Honestly, the mammary cancer stats are not clear cut, and they're not the only thing to consider. I looked into all this before spaying my older dog, particularly as a friend's dog had just been treated for it.
If you dig into the Hart et al study I mentioned above and which @Ivedonethisthreetimesalready has now linked to, the rates of MC are low, though they do say that it's a late-occuring cancer and perhaps if they had an older sample they'd find more of it.
The other cancers they looked for occur earlier in life and some of them have a poor prognosis. In some breeds, early neutering can increase the risk of joint disorders (hip dysplasia etc).
Your issues with delaying spaying are pyo, phantom pregnancies (not every bitch will have this), particularly in some breeds), the mess and the risk of pregnancy. If you have hard floors, the mess is minor. I live fairly rurally and have not had a problem exercising my dogs (out of the way places, out of the way times, longline).
If you choose to delay it, you might (depending on breed) reduce your dog's risk of certain cancers, urinary incontinence (there is a high risk of this in some breeds) and joint problems. You will also benefit from a dog whose skeleton develops properly and who matures mentally and socially. Smaller dogs mature faster, on the whole, so their minds and bodies get there more quickly and early spaying doesn't seem to be such an issue for them.
It's a balancing act with few clear answers, especially if your breed isn't on the Hart et al list.