IMHO-IME:
I believe that cube speakers are useful, it's not like just having an EQ on your bus emulating the cube, it sounds very different, more agressive and detailed (all other frequencies above and below are gone). I have a Mixcube also for mono check, and it has a different perpective on the mix, so once you hear the music trough that magnifying glass, and taking into account that it will sound a bit harsh than things actually are, it gives you information to continue working on your "flat" speakers.
I also think that learning how to mix is learning how to hear. That process, just like it is when you go to music school, can be a bit complex, and some days you'll hear things in a way, and another day you'll hear them different. Some days you will be able to focus more, and some other you won't. So it's a process of learning how to work not only with the gear, but with oneself. And there's no chance you'll escape that, it's a fact. One thing i've learned here is that our hearing perception is subject to a lot more than we realise.
So, to some people it is a useful thing to use a mixcube FOR MIXING, and other's may not like to use it, and both are fine. Or maybe they'll use it for some time, learn something on them, and then sell them, it all also depends how many hours a week you'll be working and learning that craft. Every person's life is different. There is a lot to be said about learning how to hear, a lot. Non musicians/mix engineers also have diferent interests and uses of speaker technology, and that's fine too.
Me, having the mixcube for two years almost, i still check on them. As i get better and realise how to get sounds sounding "good" (balanced), i use it less, but still use it. It's a mono check also. Like someone said, it's a "confidence" check, and that is a good thing to have. Other "confidence check" gear I use are Audeze's LCD-X and Sennheiser HD650. I know I won't be spending more money con "confidence" check gear, that's for sure. I feel confident enough to continue to work like this till i've got the cash to get three-way 6-8k speakers, I CAN TELL YOU THAT. Not worried about my work not translating, just trying to get better at it.