Difference between Aux, and Bus...

You can use effects on Auxes. There are a variety of reasons to choose one over the other, but I think most notable is the fact that on a bus you can specify the amount of a channel's signal you want sent to it. Reverb is a perfect example. If I wanted to apply the same reverb to multiple instruments, I'd toss it on a bus and send each instrument to that bus--that way I could change the amount of reverb I'd get from each instrument even though the settings on the bus itself are the same. If you had the reverb on an Aux, you'd have no control on the relative amounts of reverb on each instrument. The main uses for Auxes are, as you mentioned, different voices on a multi, or instrument groups (i.e. getting a good balance on several drum mics and running them to an aux so you change change the level of the whole set with one fader).

The term bus in Logic is a little ambigous in fact you can use it a s a bus and as an aux send.

So it's right to use a bus as a send channel.

cheers

rob