I can't speak for battery but people use their mpc's instead of s/w because:
-It is more portable. You can take it out and play live gigs with more ease and the rugged design will ensure that your mpc will survive the live environment. Also it can withstand the electromagnetic interference from live rigs and lighting much better than your average laptop, and won't crash as easily.
-Specialized hardware. The mpc is made for beat making and nothing else. While you can't browse the internet or play Tomb Raider on it, it does not have to concentrate on anything else either. Also it has wonderful AD/DA converters and sounds fantastic (in my opinion). That kind of hardware costs money, even in the computer environment.
-It will last you. If you have a computer you will find yourself needing/wanting to upgrade it before too long. Maybe you will with your mpc as well, but you'll be pleasantly surprised by it's re-sale value at least. As long as you treat it well.
-No bs interface. Once you have learned the ins and outs of your mpc, you can concentrate on making music. Not do a virus scan, or figure out why your sound card has suddenly decided to click along, out of beat. Also some times simplicity is better than a million options. It helps you focus on making music instead of loading plug after plug helplessly looking for something to just work.
-It's super fast. Record a beat, load up some carefully selected drum hits, do some overdubs, drop a bass line, erase without ever stopping the sequence, etc. -- in a few minutes you have an acceptable backing track. Where you take it from there is up to you. I have done gigs where I start with a blank sequence and just start composing. It is always interesting.
See, there is a whole philosophy to owning an mpc, and it is something to celebrate! Anyone can do music on a PC, but put that extra M in front of it and you are special. Not everyone likes it, and you can't do everything music-wise, on an mpc. But when it comes to being fast and looking cool, you can't beat em.
On the other hand, if you just don't gel with your mpc and feel more at home with software, go for that. There are programs out there which are quite intuitive and fun to use too.