Most players I've seen, take holocaust on their terminators virtually every time, and they always take it on the Brother-captain, not just on the squad. The reason is because yes, it is a risk, but it's also a chance to beef up a combat result (remember, it adds to combat results!) and it's a huge template which only has to touch it's source, not center on it. Basically, it's a way of taking a poor combat phase and adding a huge chunk of extra wounds to the fight.
For example, 5 Terminators vs. 20 Gaunts is a good fight, which you could lose. But if you average out well in combat and then blast a good holocaust near by, you can claim the life of several more gaunts that weren't even attacking yet. That means you killed more, won combat probably and denied more attacks that would happen later. It's well worth it many cases.
Losing some of your own guys is a tough one, but generally it's worth it. It's all about risking it and coming out on top, but generally it's always helpful as long as you're using it. If you don't use it, of course it's not worth while

Cheers!