Glad to hear that Polly is through her spay surgery and on the way to recovery!
One other benefit I have heard of spaying before the first heat is that the risk of breast cancer later in life is reduced significantly (from relatively common down to almost nothing, I believe). I think the risk also goes down quite a bit if the spaying is still early even if after the first heat - and either way it is another thing to think about. And of course cancer and other diseases of the reproductive system that is removed becomes a complete non-issue.