BOTH.
FOR:
If the system isn't stable, then it isn't stable, it's that simple. A lot of people forget that Prime is a real world application not a synthetic test like IBT, if it fails in Prime then it may well fail in F@H or BOINC or Bitcoin. Also just because it is stable in Prime doesn't mean it will be stable in everything.
Personally I prefer to tune my system to be as stable and as fast as possible, without sacrificing either.
AGAINST:
For some people "stable enough for what I do" is enough, playing [game] 250MHz faster than you could while completely stable and getting the weekly blue screen in exchange is fine for some people and I know a bunch of them.
A lot of people don't realise that P95 is a real world application.
Something I used to use a lot.
Those forum members involved in DC (Distributed Computing) will know what it is for.
Those with a lot of BOINC projects attached will most likely have done work for PrimeGrid which uses the P95 (GW NUN) math libraries for the majority of their projects.
Before using a PC for any of the DC projects it was common practice to run P95 to prove some level of stability before doing real work.
Returning bad results helps no one was the mantra.
Having a P95 stable computer meant that it was also very stable at normal everyday tasks gaming included.
Things have changed a bit since the AVX CPU's become available.
Using the AVX instructions speeds up number crunching a lot and consequently DC computing and encoding APPS are using it a lot.
Games do not make much use of AVX and testing with AVX enabled P95 may not test the integer and standard FPU instructions that games are using.
P95 used to, but not any more.
Generally the AVX instructions will start to fail at lower clocks than the traditional X86 instructions but that is not always the case.
P95 can still be useful to a gamer as it will quickly weed out very unstable CPU and memory clocks.
No need for long punishing tests though unless you are going to get involved in DC or are going lone wolf to find the worlds biggest prime
Gamers can use looped benches such as RE6, to test stability.
A favourite of mine as it is sensitive to instability in the RAM or CPU.