In reply to ElvisMax1:
I have been bringing just 4 (I think the #5, #6, #7 and sometimes #8 WC hexes) - and don't bother with the others (I rather have offset nuts than small hexes, and I have never used the #9 hex or been wanting for it). I have these racked on single crabs so that I can use them without quickdraws (most of the time anyway).
I do find them useful, as you get 3 ways to place them. I have also heard that using cams in your belay setup is not a good idea for two reasons - 1) you've used a piece that is better suited to being placed on the lead, so if you're in a tight spot later and want a quick bit of pro, but are missing the crucial size of cam... 2) cams can walk when loaded in the manner that a belay usually is (you might shift slightly and tug it gently every so often). Also I just prefer a solid hex to a cam for crucial placements.
The last time I was climbing, my partner wasn't sure if we should do the route as he didn't think there was a good belay anchor at the top. Well I checked it out and judged that there sure would be, and I'd have to use the #5 hex as a key part of it. Well when I got up there, I realized I'd left the #5 deep in the rock in the middle of the pitch... There wasn't much else to go into that spot of rock (a cam stem would've been too fat to fit down the gap). Then I noticed the #6 almost fit... I grabbed a rock, and gently tapped the #6 into a lovely, snug fit that couldn't have been more perfect.
You can also hammer hexes into verglassed rock during winter (whereas I don't know what you'd do if you only had cams - spend time scraping all the ice off with a pick?), or if the rock is just a bit mucky.
Lastly, hexes are lighter than cams for the same size of protection, so if you know the rock will accept hexes - it could make sense to have hexes on your rack even though cams would do the job.
Post edited at 13:32