for all revo owners: fail safes are stock and don't go out and buy a part you already have.
It think that is a bit too broad a statement Mach1, while is does appear the 3.3 has one, the 2.5 may or may not, hard to say without looking at the manual or getting word from Traxxas directly in some other way.
At the end of the day I think it is personal preference, some people like $1M of life insurance while some think $500k is more than enough. Armed with the information that the 3.3 at least has some level of failsafe allows each to make their own (informed) decision...I won't be adding one unless I can find some new information as to why the built in one is "risky".
Cheers
HW