It's funny though, even though I don't make a lot of sales, the majority of my sales come from the sponsors with low length or no cookies.
So based on that, I thinking that it all depends upon how the site converts rather than if the punter revisits and joins at a later date
But like I said my traffic is low and sales are peanuts... so if you have high traffic and lots of sales, I would be interested to hear on how it works for you (runs off to look at Greenguys stats).
Thinking about it though, there could be 10 million other factors involved in the process - still it would be interesting