I fish with a flyrod almost exclusively, but when its not the right tool, I'll pick up a spinning or casting rod.  Depth and wind are the biggest reasons I put the flyrod down.  

Depending on you and your desires - and the time of year  and species involved - you might take one. Or even only a fly rod.

Unlike most lures heavy enough to cast (floating plugs excepted) you don't have to be working the reel and rod all the time. This means if you're fishing 2 in a canoe, the stern guy could be fishing a flyrod and still have plenty of time to maintain course.  

Casting a popper out and letting it sit for a loooong time before moving it is a great way to pick up smallies.

With a sinking or sink-tip line you can get a fly down pretty deep if you're trolling, or if you're patient you can strip a fly in pretty deep.