If you are easily distracted, working in an accountability area (as in no YouTube) helps get you focused - at least for me. If you work as a team, I don't think you can replace being there very easily - whether that means your apartment or getting an office.
I work full time for another company (got to have income from somewhere) and whilst I work my other interests I figured the office would be a good idea.
I have so many projects in my head and my time seems to get occupied so easily with stupid things (youtube, hackernews :) ) that some office space seemed like the easiest way to stop that.
As I mentioned its more of a personal choice I would imagine, trying to see if I have considered all the factors or anyone has any cheaper alternatives.
