If you're just getting started, and you have no leverage, picking up part-time founders makes perfect sense.
They will never be as devoted as you will be. They will never be as committed. Sure they may be talented and will write tons of code for you - but what happens if they leave and you have to figure out how all of their code worked?
I'm sure you've read all the articles written by other entrepreneurs about how much dedication it takes to build a company. There are always exceptions to the rule (del.icio.us), but I'd say most successful startups require your full attention.
Vest everyone's shares too - if I bail on you early, I might just have a few % of the company when I leave.
I think it's OK to have varying levels of commitment. Of course, the CEO should be 100% in the game. But I've worked with developers part-time, as partners, and found them reliable and committed with a lower number of hours.