I wasn't so much thinking about the funding provided by YC; but rather that if you've reached a stage where you can raise enough funding to hire yourself into the US, the fact that YC provides connections to angels and VCs is probably not tremendously useful.
I think you have the order of events wrong. The connections (and other value) YC provides help you get that funding which lets you hire yourself into the US.
As pbiggar points out above, there's no problem (at least for EU citizens) getting into the US for three months of YC - it's staying there long term that's hard.
I assume for visa waiver countries those 12 weeks can be done in a 3-month visa-free visit, with the purpose of "seeking investment" or so. Otherwise you'd have missed the cycle after going through the "annoying and long" process of getting a visa.
It is possible to bootstrap a company in such a way as to plateau out at, say, 3 people, each of whom gets a decent market salary. However, that company may not have further room to climb for various structural and/or marketing reasons, lack of appropriate connections, funding opportunities, etc. In principle, the purpose of YC is to address that specific gap irrespectively of how much short-term revenue the startup may be generating.
On the other hand, I agree that the amount of funding they provide per se is not particularly useful if you are already generating enough revenue to pay yourself(ves) a market salary.