When ever these threads pop up get get two inevitable response groups.
A. People who have it on for many reasons.
B. People who have it off for a handful of reasons, but mainly nostalgia i.e. grew-up without needing it.
Group A has problems, but they're easy to fix. Group B has problems - though fewer as they're more technically astute, but tend to profess they've never had issues without it. It's like saying you don't use a seat belt and have never had an accident until the day it happens and turns out to be permanent.
MS have gone to great lengths these past 5 years to help people but it always comes back to someone reading some blog/determined to improve what they can then "tweaking" (i.e. turning stuff off) until they break it.
Incidentally MSE is indisputably one of the best av around and consistently outperforms most paid for AV without the privacy/wallet invasion that comes with them. Probably has something to do with MS engineers actually knowing what should and shouldn't be happening in windows
MSE is built into windows 8 for the record, it really is that good. Unfortunately some people will still remove it in favor of the more familiar, less effective, more intrusive, paid solutions. That's life though. Sophos, highly effective industrial AV (like Cisco uses them effective), hand out their mac av for free trying to help these people but some are just determined to make things worse for themselves.
If Darwin could see them now...
.
and then when perfectly good explanations are given back they throw their toys out the pram.
) (*touch wood*)