For plastic worms... M or MH/XF
For Small Cranks and Small Topwater... M/Mod
For Frogs... H/Mod
For Large Canks and Large Topwater... MH/Mod
For Spinners and Buzzbaits... M or MH/F
For Swimbaits... MH/Mod
For Small Jigheads... ML/F Spinning
For Evertything else... Mix and match or a M/F Spinning rod
That's what I use. I don't think they are at all necessary, but there are advantages to each that just might get you hooked-up instead of missing a strike. The fast and extra fast tips are great for quick hooksets, the moderate, softer tips are good for allowing the bass to have an opportunity to take the lure without snatching it away.
But, hey... it's just my opinion. What do you think?