1) You oftentimes can't afford success. Google and Facebook would've gone broke without outside funding of some kind, long before they found a way to generate significant revenue.
2) VCs are advisors, and are likely smarter than you about lots of things-- especially how to sell a company. If you want a lifestyle business (37Signals, for example), then avoid VC. If you want to sell a company, you'll have a better chance (and sell it for more money) with a motivated VC.
3) Sales, marketing, and support cost money, for most businesses. If you've got paying customers, telling them "sorry, I'm at my day job" just won't play. For a consumer play, this is less of an issue.
4) A lot of ideas (b2b, mostly) just aren't simple enough to bootstrap with 3 people and 18 months. Of course, it's easy enough to avoid those business ideas. ;-)
5) Businesses are ticking timebombs. Stretch out your development time and your stretching out the opportunity for disaster. A team member can lose interest or get a great job, a killer competitor can manifest and snatch up your customers, etc.
6) Momentum. I can say with experience that it's REALLY hard to keep momentum going with 3 people working part time (I'm doing it right now). When you put a startup on the back burner, it's rare that you all get to focus on it at the same time. When you're rarin' to go, your partner is "really slammed this week".
7) Necessity drives success. When your startup is your full-time job and you have an investor looking over your shoulder, your going to work harder and better. If you've got a job, startup failure is more of an option and delays aren't very painful.
8) Debt or VC can get you to the point where your growth curve starts quicker. When you are confident about the fact that you're on track to build a zillion dollar business, $100k of debt/equity financing get get you there a LOT faster.
All that being said, I've sold two bootstrapped companies-- it works.
I'd advocate for bootstrapping your way to the point where you know if you're on to something. If you can prove that the market desperately wants what you're building, then it USUALLY makes sense to get some cash (via debt or equity)... Assuming you want to focus on growth/exit events instead of profit/lifestyle.