Unless you have quit your full-time job, you have no idea what it is like to be truly committed to your idea. No idea whatsoever. I would say "lack of conviction" is an understatement for cofounders that have not quit. They have one foot on the dock and one foot in the boat. When the boat begins to sink...they just lift their foot off the boat and forget it ever happened.
If you can't make payroll, you offer no health insurance, and you have no money, your "full-timers only" rule locks out a huge portion of the talent market. People with talent and experience have better startup options than you seem to be acknowledging.
If you're just getting started, and you have no leverage, picking up part-time founders makes perfect sense.
I'm assuming we are talking about a blend of full-time and part-time cofounders. Let's say you are the full-time cofounder. Before you take the plunge, you better make sure you find out if your "part-time" cofounders will ever plan on quitting their full-time jobs. If they say no to you, will you be prepared for the day they decide to quit?
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.
An easy solution is to have uneven ownership. If you're quitting your day job to work full-time on something, and I'm drawing a salary and working 20 hours/week on a side project, I'll expect you to be a minority partner.
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.
There are no "decision making rights" in a 3 person startup. Anybody can disagree strongly enough to leave and kill the team. Leaders lead. They don't dictate.
