depends on what you play mostly....if it's indoor/tight-ish CQB, the a secondary (i.e. pistol) is a great option.
if you're mostly skirmishing/gaming on fields...then a backup rifle is a solid piece of mind (not that I'd carry two rifles out...I'd just go back to the staging area and grab the other one it the first one died). It's best to have them take the same batteries/mags/etc...but that's not a necessity. It's sometimes just plain fun/nice to shoot something different every now and then.
OR...if you say had an M4-style main rifle already...and mags for it...then you could whip up a patriot/tanker style short M4 platform pistol and sling that as a backup with you. Same mags would work...you'd probably end up with a different battery, but even then maybe not if you went for a stock tube lipo setup.
Such a thing wouldn't be all that comfortable to use as a pistol...nor would it be great as an aimed SMG...but as a secondary "f*ck off" weapon, it'd be sweet.