The filter debate is one that will always rage on here. Personally, I have one 77mm UV and a 62mm UV, which covers all my lenses when you consider step-up rings. I only use them in harsh conditions. Usually, they don't affect image quality, but if I turn slightly into the sun, they will increase flare badly. I don't want to spend the time removing and adding filters every time I change position. 99% of the time, they offer no additional protection from the hood, and aren't needed, so why take the reduced image quality and increased flare?
Easy to slap it on when needed. Also, for those who say 'it protects me from losing money.' If you have 10 lenses, and buy 10 quality filters, that's $1,000 or more...about the price of buying a new lens. I have one per general thread size for blowing sand, salt spray or heavy snow and rain, and I have good insurance should a very expensive lens be somehow destroyed.
Frankly, in normal shooting situations, the chance of the front element being scratched enough to affect image quality, while using the hood are practically zero.