Will you be able to take public credit for your contribution, and will it be big enough to impress people? What I'm thinking here is that if they become very successful and well known, and you can point to them and say "I'm the guy who built their infrastructure" or something like that, that might get you a really good high paying job somewhere else or get a good consulting career launched.

But generally, unless you have some specific plan like that in mind, I think the vast majority here are right and it is a bad idea.