A large part of the reason Dollhouse was renewed is because Whedon made "Epitaph One" for half the cost of a regular episode by shooting on digital video and using other shortcuts. That demonstrated that maybe he could bring in a whole season for a greatly reduced budget. The budget could be cut more by abandoning the limited-commercials format of the first season and dropping the length from 50-plus minutes to the more conventional 42. If it doesn't cost as much to make, that means its ratings don't need to be as high to break even, so it's feasible to keep it around longer. (Also more commercial time means more income for the network.)
There's a potential problem there, though. Is there not the possibility that more commercials means more people will DVR it and thus lower the overall number of people who watch the show live ?
Some of the most expensive advertising slots on British television are at roughly 4:03pm on a Sunday on Sky Sports 1. Why ? Because that week's featured Premier League match kicks off at 4:05pm.
Those slots are always filled with ads for high end products - HDTVs, for instance, and often one company will buy the entire two minutes. It's all because people don't want to go anywhere for that short break between the teams lining up and kick-off.