Another issue is that you don't want TOO great a vehicle lifetime, unless you're conducting long-baseline experiments like climatology or seismology. The reason is that if you launched 2 dozen MER's, and had say 75% success in a range of landing areas from easy to crazy, then you'd have 18 sets of telecoms to manage through the DSN.

I guess you could give them away to anyone who had a 35m+ dish and was prepared to manage the comms, but you'd still get a lot of confusion and cross-talk to and from Mars...

I guess, though, if you knew having too many was a problem, you'd be prepared to do risky things to try and get to neat places or collect key data from hard-to-reach places.