The serious use case for these miners is that you have some kind of incentive for your users to mine, not just have it mine randomly in the background. I guess it works for pure donations but take a look at "Use Cases" on the front page for a couple of examples of what I mean.
I mean it's not like it'd make sense to flame blizzard or bethesda because you can't play as long as you'd like on a laptop/mobile.
You use the proper device for the proper use, and this way you just get an additional option to trade value with the site owner. You're free not to mine in the same way you're free to get a paid account if you don't like ads.