I fly the airways a few hundred days a year. The type of situations in which Jerricho and Scott mention are few and do not represent the vast majority of us who understand the importance of controllers. To bitch about the occasional pilot who bucks the system is no worse than us bitching about the odd controller out there that tries to fly the aircraft for us. They are out there, but seldom seen and of no real relevance to the discussion about the importance of TCAS. If however its therapeutic, have at it.
As Phoenix said it, its that 1 in a 100 theory. What if that guy you believe is going to level off 1000 ft above doesn't? What if he/she has some sort of vertical mode selected that doesn't allow alt capture? Dialed in the wrong altitude in to the alerter...what if....what if.
Many companies here in the US and I'm sure in the UK have safety programs that allow pilots to submit reports on screw ups. The product of this is a database that one can read through and try to garner some degree of insight in to the chain that caused the infraction to begin with. Suffice it to say the folder with altitude busts is the thickest.