Funny that you mention it since I'm also an avid Clicker Heroes fan.
I agree that games like this can be very addicting too, but there's also a slight difference: idle games can be played for 30 minutes a day and still reward the player with steady sense of progress (which is what idle games are all about after all).
Most MMOs on the other hand, like the ones that I mentioned, hook you up with quick gains at the beginning and then slowly drag you into more and more activities that take more and more time. At the end of they you end up playing several hours a day just to get this one level or some such. I guess it's a strategy to squeeze some more monies from players by offering a way out, like boosters and such.
Another thing, even worse: some MMOs lock up certain features behind paywall (LOTRO) and then encourage you to spend cash on unlocking them, which is even lower move than the one I mentioned before.
Yeah, the more I think about it, the more I start to dislike MMOs.
The problem that *I* have when playing those games, sometimes, is that I want to play close to optimally, and to do so, I need to be at the computer at the right time to make my next move. Essentially, this means I'm constantly thinking about my progress in the game, and most significantly, I may be pushed to the point of re-arranging my schedule, or at least changing my sleep schedule, just to do better at the game, and it's hard to get out of that mindset.
For some incremental games, those with more active oriented gameplay, it's reasonable to turn off offline progression and actually turn off the game, and I find that that makes me not worry about missing things as much.
The issue is largely psychological, but it is definitely something to be aware of when playing or developing incremental games.
(Worth noting that incremental games are much easier to make than MMOs. With that said, incremental game development is still serious game development; the fact that balance issues can crop up us one example of this.)
Yes there's plenty of micromanagement involved in incremental games if you want optimal progress, or at least not fall behind too much. Use of calculators to better manage your game can take quite a bit of weight off your shoulders, though it does take away from the fun of personally designing your setup which is half the fun of those games after all. In the case of Clicker Heroes there are also whole mathematical essays on Reddit about how to optimally micromanage your game that help a lot, and are also tons of fun to read.
I remember two games that used to keep me up at night like that: Tribal Wars and Travian, though I didn't play those in a long time. By the way it brings me to another form of games that are kind of designed to be as addicting as possible, namely browser games. I used to play lots of those in my youth, fortunately I didn't have much money to spend on them then because I most certainly would have.
I'd say there are even more mathematical problems than strictly game-design ones when developing incremental games. As you said, balancing such a game is most certainly a challenge.
All in all MMOs, incremental games and browser games are all designed to be addicting, unlike single player games, especially story-driven ones, which are often more akin to reading a book or watching a movie than spinning the wheel of fortune. Still, I don't think devs can be blamed for that - money does not stink after all.