One thing I saw is that MSE does fine for a very basic protection, but doesn't have a lot of the other options other programs have. If you don't want anything else, it's fine, so it'd probably suit me, but I still like having some other options, even if I avoid most of them. In addition, it seems like they were one of the top, but... Couldn't be bothered keeping up? With the amount of competition, it probably wasn't worth their time and effort to keep on top of a free market.
That said, some of those "options" include tool bars, safe search filters, and other such nonesense that you really don't want, need, and slows things down. Things that the browser itself often does to a certain degree, so I make sure not to install those with the program.
I switch every couple of years because everything is good at first, but then factors change:
Car Insurance - go with one, and is fine for years. It'll continue being fine. But if you call your company up, and say you want to switch, they will often offer you a discount to keep you. If you search around, you're likely to get a good deal to join with someone, and slowly, their price will go up. You won't really care, because you've got it and it's inconvenient to look around, but if you did look around, someone else would offer you the same, if not better.
Electricity, gas, services all do the same thing. Even a lot of mechanics - they lure you in with a cheap service, but once they know you're coming back to them, they find one thing, then another, then another - things that a new mechanic (trying to get you to return to them again) would say didn't actually need doing. Unless you have an awesome mechanic.
In terms of anti-virus, I've used Avast, AVG, NOD32 (*cough*), and Avira - the big ones, and they're all fine, but... When they update their build, they often sneak something new in. Something stupid. They throw in Tool bars, Browsers, adverts, popups for their other programs, build updates that don't update automatically and keep popping up, safe searches, filters etc. It's fine for a while, because you can avoid most of it, but then there's something you can't eliminate, or just becomes a challenge to deal with.
I avoid their additional options, but AVG Free 2015 has made it so that the update option popped up, just after installing it. Why would I need a new build, when I just got the latest? It may've been bad timing in that they just put out a new build, but you can't turn off that type of popup, and it became annoying.
Avast Free now has a pop-up that advertises other of their programs. I don't know how often, but if I see it, it'll be gone as well.