Well Rynnas signed here because he wanted to work with Allaire. The best he was looking at was a backup position in the NHL; instead he'll be #3 on the charts in Toronto, but the benefit is that he can spend most of the year working with Allaire (which I'm sure was THE selling point).
Outside of that, Gustavsson looked very, very good after the Olympics, but we can't just pretend that the first half didn't happen. He was solid, but inconsistent and there is no say that he will ever become consistent enough to be a starter in this league. Aside from that the Leafs have a proven guy in Giguere who might be on the downslope but who isn't signed beyond 2010-11. And the only other option in the organization is Reimer, who is basically a second rate prospect (albeit one coming off of a tremendous if abbreviated season in the AHL).
The Leafs need a goalie and Rynnas gets to come over in a lower-pressure situation and without any expectations on him. He also gets to work with a world class goalie coach, and one that he clearly wanted to work with.
It's a win win. And it makes sense from all angles.