I think it really depends on your situation. I for one love using donuts on training. Aside from using them to run training while you sleep, you can also use 2 remotes and save yourself 50 minutes. For me, this means knocking out 1.75 hr.s a day for the cost of 10 donuts. Now, I make a minimum of $65 an hour when working. 1 donut saves me 6 minutes of time on missions as well. So 10 donuts there saves me an hour. At their most expensive, donuts cost $2.99 for 19 of them. That would save you about 3 hr.s of time spent on the game, unless you're willing to jump into the game every few minutes. I'm not. I would rather spend a couple hours a day doing 2 minute missions and spend donuts whenever a 6 or 12 minute mission pops up. Now if money is tight and time is abundant, I understand not using donuts on missions and training. But for those of us who look at how much time is spent vs. the money we can make in that same amount of time instead of focussing on the game, it becomes more equitable to spend donuts to save time.
Personally, I can get all the missions done and all the training in just a couple hours, leaving the rest of the day to spend w/ family or working. I get 340 energy with trophies and all energy refreshes. I also get 25 total training units with trophies. That comes out to 7 hours and 45 minutes of time spent going in and out of a game, which disrupts the more important things I need to do.
By spending donuts on training 20 of those training units and using remotes to end them quickly, and on all my 5-6 min and 11-12 minutes missions, I can knock the time needed to pay attention to the game down to approximately 2 hr.s.
Now add in a villain that lasts between 1 and 3 hr.s and the 25 energy and 7 training I usually average on them for an additional 1 hr. spent paying attention to the game. That would bring my total time paying attention to a game to 9 hr.s and 45 min to 11 hr.s and 45 min.s................
That's nearly half a day spent paying attention to a game. Sure you don't have to sit there the whole time, but if you're jumping every 2 to 12 minutes, you might as well be.
Personally, I'd rather spend an average of 4 hr.s to do everything that can be done in the game in a day, minus fights, than an average of 10. During that 3-5 hours, I can sit there and chat with friends and teammates. Then when the time is done, I can go about the rest of my day and life.
$50 easily lasts me a few weeks and takes less than an hour for me to earn. Even at $10 an hour, you can buy 75 donuts on the facebook site and make it last for a good half a week. And that's on the more expensive facebook site. On the stand alone site, you get 100 donuts for the same price. So for the cost of 1 hr. of work at $10/hr. you can effectively save yourself tons of time in game.