I typically let my beer sit in the secondary for some time while I wait on 50+ bottles to become available. If some yeast is stirred up when I lift the secondary up onto the counter then I figure it's fine - it'll just help the beer carbonate a bit faster. Besides, yeast will also settle out while the beer is sitting in the bottle.
To answer the original question:
A secondary will allow a surprisingly large amount of yeast to settle out - you'll have clearer beer. It also affords you the opportunity to "dry hop" your beer - this is very popular and some would argue necessary for IPAs. Lastly, bulk aging your beer will allow it to mellow and it will age more effectively in bulk than it will in bottles. So in short, yes a secondary is a very good idea. While it's not imperative that you have a secondary it is one of those things that will definately contribute to quality beer.