Aux returns are just handy "extra channels" without many features that are especially useful for plugging in things like CD players, cassette decks, turntables, video decks, etc.
People think that because Aux Sends and Aux returns both have the word "Aux" in them, that one is somehow morally obligated to use them together. Not so. You can if you want, but there is no real advantage to it.
Many people almost never route an aux send back through an aux return. I use aux sends as headphone or monitor mixes a lot, and those that I use as effects sends I return through regular mixer channels - that way, as Farview pointed out, you get the full control of the signal (panning, EQ, insert points, additional aux sends for sending the fx to cue mixes, etc.)
A good quality mixer may have 6, 8, or even 12 aux sends. But maybe only 4 aux returns. Aux returns just aren't very important - there is nothing you can do with them that you can't do with a regular mixer channel. The only time they become really crucial is when you have a really small mixer, like only six channels, and then the aux returns may be critical because you need every input you can get.
Once you have a board with 24 channels or so, chances are all you will use the aux returns for are to keep a CD player etc. permanently patched into the board.
