I wouldn't dive my primary without one, there are issues and things to remember about them though
1) when molding be very relaxed, you want just enough imprint to make the hold tight, and I found if I press outward with my tongue really helps them to get a firm and comfortable grip on my teeth.
2) If you trim their length, do it in small increments after its molded, cut too much, even 1/8" too much and they are worthless, think about how annoying a grain of sand is in your mouth, how small is that, cut carefully
3) I found that they are worthless if you have to share air with your primary second stage (not exactly DIR compliant), Also your buddy with be annoyed if you have an Air2/safesecond and he/she is stuck with your molded mouthpiece, granted they aren't exactly going to complain while being rescued, but it one more thing that might stress them in an already stressing situation.
4) I have also found they wear quickly, generally by the time your reg is ready for its annual service, they have hardened (lost elasticity) and will often crack when removed from the reg. Not a huge deal, but $29 extra a year to replace. Although, the newer, Hi-flow ones seem to be better about this (only ones available now, I believe), I don't think my last one cracked, but I think that was a first in about 6 years.