The main case that 1-3 month trial periods don't work for is when the person you're considering hiring already has a good job, esp. one that they like. This is often/usually the case when you're talking about top tier talent. Want to try selling a stud on coming to work for you for a bit for a chance at a real job?

Even if it's a tiny one-week project. It would be helpful to know everything about how they work before you gave them equity.

That's why you have vesting.

The event was "Startup Hiring: I've hired all my friends - now what?"

http://www.finance4founders.com/2009/10/19/startup-hiring-i%...

But it was an excellent post, and the slides are worth looking at too.