For me, the dual mode is "get me the best you can", and 2g is "save power please".

With my operator, dual or 3g, when inside 3g coverage, drains the battery in about 4 hours whereas 2g lasts days, so I usually have it forced to 2g.
Now I don't know before switching to dual or 3g, whether there is 3g signal available, and sometimes it takes quite long to acquire 3g even if it's available, so I've previously picked Dual Mode from setup, as this automatically falls back to 2g if there is no 3g, and doesn't leave me offline.

And of course, if you only live and move in areas where there's always /some/ 3g coverage, however weak, you might prefer to lock it to 3g.. There are use cases for all 3. :/