As someone who has at this point opted not to do it, I have four reasons:
1. Cost
2. Nullifies investment I've already made in holsters (see no1)
3. No known ankle holster for the P3AT with armalaser
4. Simplicity (one other thing that could possibly fail, and must be maintained with fresh batteries)
Now, this is not saying the armalaser would be a poor choice for anyone else...in fact, I thought it was really cool and if it weren't for the cost factor plus the holster issues I would go for it.
If you offered me a P3AT with an armalaser, plus matching holsters (pocket and ankle), or the same thing without the armalaser, I'd take the laser equipped one. But right now, I can't afford the unit, much less having new custom holsters made (the ankle one being the trickiest!).