Mark,
A Couple of reasons for them -
The varying lengths of tee shots means fw bunkers probably should be extended at least on some holes. If you place them at only 290 off the back tees, they don't come into play for very many folks, even with multiple tees. On the Cog Hill thread, I was going to mention that Rees, in his tournament work, seems fond of long bunkers or clusters to make the strategy similar for varying length tee shots.
Once you have decided on a large bunker zone, either for reasons above or for generally large scale of the project, its a lot cheaper to leave some grass areas in between, rather than build a solid sand bunker, especially if liners are used. Typical SF cost is about $5 SF now. Leaving half the area grass means its not a budget buster.
Lastly, I like variety in bunker complexes and generally build a cluster set of bunkers, one or two over sized, a few standard single bunkers, etc. Visual variety is important.