As a sponsor, of course we'd love to see an affiliate work only with us... but on the other hand, if you're working with multiple sponsors, you not only spread your risk, but you potentially have that much better a chance of making a sale if you offer a variety of choices in the same niche.

If they don't subscribe at this site, maybe they'll go to that other site offering similar content and sign up there... and done right, it should increase your overall conversion.

The flip side is I think if you choose *too* many sponsors, you run the risk of confusing the surfer so he can't decide what he wants and might end up getting nothing... so matching up the sponsors with the interests of the traffic and targeting what they want specifically will probably get your best results.