I voted "it depends" although I guess really it would be a "can't live without them situation"
I do have to turn them off to run Metro's challenges though, and that is the only time I play without them now.
However, when I say that I find them essential, its mods like Twallan's NRAAS Saver, Overwatch and Error Trap and a few others. The reason I find them essential is that even with a cleanly installed game, my Sims games break within a few hours of playtime without them. For me it isn't an option, its having to use mods to fix things that EA should have fixed themselves. Without Overwatch to remove stuck sims, abandoned cars, etc. any save game becomes unplayable for me within a few weeks of sim game time. Doing Metro's challenges for me are so frustrating as I can guarantee my game is close to unplayable long before the end of the challenge time frame. I also have to use a mod to allow several items I purchased from the EA Sim Shop to work. Without it, items like the Tree of Prosperity just sit there and do nothing for me. And these things happen regardless of whether or not I choose to use CC from other sources. Even on a fresh install with only one or two shop items its the same for me.
And this is with a computer that happily runs new games like Diablo 3 on best settings very well.
I've spent years tweaking settings, doing factory resets and tried every single suggestion I ever found on our technical forums (yes, that took forever). In the end, my choice comes down to running the game with a small selection of mods to fix it when it breaks, or never get to play. I would much prefer it if I didn't have to use mods for these reasons and could just rely on EA's game alone, but if it hadn't broken every single time I played it, I probably wouldn't have gone looking for help and found you all in the first place!
I do sometimes use mods to do fun things like story progression for example, but these I only use when I fancy a bit of a change. These are what I assume are the sort of things that most people are thinking of when they refer to mods. These I don't consider essential, I think of them as an interesting twist for a personal fun game.