Just 5 minutes browsing through some random WordPress blogs, shows an alarming amount of sites that use no kind of on-form filtering from bots!
In my experience, spam filters can do a great job but can also take up alot more resources, then actually stopping bots from the start.
My earlier WordPress sites used mainly spam filters but I quickly found how many times my site was being hit. 3000+ times within a couple of months… seriously. Bots now making up the majority of my reader base, imagining the work that the poor database servers had to endure, led me to believe that it wasn’t the best solution.
The invention of Google’s captcha really has changed the way sites work. Being the final stage in a comment or a login page, it can prove a bit unsightly to say the least. Unsightly, maybe, but not useless at all. Just to be safe, I still use a spam filter… but not many, if at all any, get through Google captcha.