Having recently arrived as a pilot in the world of GA I was deeply shocked to discover that it was legal to operate a small jet below 5700KGS without TCAS. It is absolutely ridiculous that any new VLJ should not have it and any operators who are doing this should be named and shamed. (I am certain that the new Mustangs being purchased by LEA are all properly and fully TCAS equipped by the way.)
In ten years of busy airline shorthaul flying I know that TCAS saved my bacon and those of my pax at least once through an RA and it is a vital tool in any event to assist situational awareness because of the display of prox traffic in busy airspace. Just think how many times one has regulated a climb or descent rate or queried an ATC instruction because of ones ability to "read" what is going on around one. It is essential.....
The problem I suggest arises more with older equipment like CJs where a retrofit costs in excess of £25000 rather than purchasing of new, where there are no excuses.
I must say that I did think that there was legislation which was coming into effect this summer making it mandatory, but I am not sure where I heard that.
The biggest problem seems to be a lack of appreciation by some owners who despite their millions are blissfully ignorant of the risk and this is componded by cowboy operators who are literally gambling with the lives of their pilots and indeed customers to make themselves a few extra pieces of silver.
TCAS should be mandatory for any AOC operated aircraft.