Another reason is that an out of tolerance transponder / encoder can give a false altitude to TCAS; I've seen this on a number of occasions where a GA aircraft has gone past at a different relative altitude to that shown on the TCAS display.
As have I, to say nothing of spurious readings that miss traffic completely, or miss based on operational practices. Many times I've watched traffic come by at FL350, a thousand feet separated, with nothing at all on TCAS...but the next aircraft to pass is right there. It happens. Why, I don't know, but it does.
I arrived over a fire years ago when TCAS had just been installed in the lead aircraft. I reported my altitude, I was above the lead, he could easily have looked up and seen me. However, he reported that he couldn't clear me into the fire because two aircraft were circling the fire directly above us, shadowing our positions, but at a higher altitude. We could clearly see that wasn't the case, but he was comparing the altitude we reported in to what he was seeing on TCAS, and making the riduculous assumption that two identical aircraft were above us, exactly the same altitude above us, on opposite sides of the fire orbit. It took some convincing to get him to actually look for himself before we were cleared in to drop.
Conversely the introduction of TCAS has been largely an improvement in that arena, where 30 or more aircraft can be working in a very small area with nearly nil visibility, close to terrain, on up to five or more frequencies, with a vastly disimiliar mix of single engine airplane through large multi engine airplanes through helicopters, in low visibility and rough conditions...the heads up provided by TCAS has proven to be both a help and a hinderance at times...and that's one place where one does NOT want to respond to an RA, because terrain always wins the conflict.
The original poster will obviously not be using TCAS 2; if he did choose another system he would presumably read the instructions and use the equipment properly, and with due regard to its limitations, just like any other piece of on-board equipment.
The point is that while TCAS II offers the best situational awareness, display, aid presentation of information, even it has limitations...especially in a particularly dense traffic area. Further, while one may be alerted to traffic and then see traffic in that area visually, one may very well be deluded into thinking because one target is displayed and one sees one target, then one has acquired all the traffic that's there. This is an extremely dangerous assumption, but one that's easy to make, and often made, where traffic reporting services or equipment is available. One must always assume that what one sees is the least of one's worries, and continue to look for the traffic that's not displayed or not seen. It's not the traffic you see that kills you. It's the other guy.
If one isn't looking, as though one's life depends on it, one IS the other guy.
Other displays which don't offer a 360 degree situational display with relative traffic positions and vertical closure rates and elevations offer precious little information and tax the pilot for keeping situational awareness on all the traffic. This is a hazard, and can be a distraction from seeing traffic which may be a conflict which isn't displayed.
TPAS and some of the other innovations do have their limited benifits, but I wouldn't invest in one for my personal flying as I don't feel they offer adequate information, reliability, or service to make their distraction or cost worth the while.
Many of us who have to sit for eight hours with the sun in our eyes at high altitudes will put up sunshades or clip charts to block the sun...which does prevent visual acquisition. And the truth is that ones eyes quickly fade to empty field myopia at high altitudes with no field definition...we're seeing a spot three feet in front of our face and don't know it, when we look out that window for traffic into the haze cloud, and distance. We have radar separation in some cases (though for much of the world, precious little), we report positions through HF relay stations, but we still watch the TCAS closely, look out the window and back in again (to prevent empty field myopia) frequently...or we should.
I don't know how many have ever been in and out of the clouds, flying IFR, and had a non-reporting airplane blow by the windscreen close enough to see the brand of sunglasses worn by the other pilot, but I have...several times...even when talking to ATC and squawking a code, and using all the resources available to me. I look for traffic when in the cloud and out, when VFR as well as IFR, as though my life depends on it. It very much does.
I'm not convinced of the usefulness or viability of the cheaper traffic systems designed for general aviation cockpits. With the increasing capabilities of the displays and avionics suites that are rapidly becoming available to the average private pilot, there's no reason that better traffic information can't be integrated into these displays every bit as much as they are on a corporate or airline aircraft. The little velcro-to-the-panel displays with an arrow and a number are very near useless in comparison, and in my opinion, detrimental. It's just far too easy to see traffic displayed, see "an" airplane out there somewhere, and think that's it. Like I said, it's the one we don't see that's the dangerous one, and we could very well be that dangerous one if we're too busy playing with gimmicks that offer limited utility and poor display information, when we could be exercising our necks and our eye sockets in a vigiliant effort to spot the other guy.
The old mexican joke is about a cab ride in mexico city, when the bichito (cab) driver blows through every red light he comes to, and stops at ever green light. The tourist finally asks why the cab driver stops at the green lights, and the cab driver replies "my brother might be coming the other way."
I spend a lot of time looking for that brother in flight...especially the one who's letting his traffic display or his radio do the traffic scan for him.
One of my biggest pet peeves, or annoyances on the radio is the guy who says "Lincoln traffic, Cessna XXXXX six miles south at four thousand, over the bean plant,
any inbound traffic please advise."
Any inbound traffic? No. Just us guys with no radio, no electrical system, the ones who can't hear you...but still have eyes to look...