Well, technically you only really need a standard 3.5mm to RCA phono cable to connect your iPod up to your M31, but (ignoring sound quality for a second) the docks have the advantage that, as you say, most will also charge your player, in some cases you'll get a remote control and also it looks a lot tidier.
is well rated within WHF, has all the above benefits, plus a video output and also that it won't continue charging the battery once it's fully charged (which can cause the battery life to reduce if done a lot). The big disadvantage of course is the price (generally about £150).
The one from Apple
offers everything except for the charging benefit of the rDock for only £30- £35 - however I haven't heard the rDock to say how much better it sounds.
Have a look here
to see the various docks WHF have rated.