There's no problem at all with doing that, if you can handle the cost. It's going to cost significantly more, obviously, than if you just took the classes during the semester (since there is no difference in cost between 12-15 credits during a semester, but summer and winter classes WILL cost additional money, about $800 each I believe).
Additionally, most students don't work as much during semester and take on 40 hr/week jobs during the summer. So, I don't know if working 9-5 and then going to class 6-whatever poses any issues/problems for you. Or, if there might be scheduling issues with that. Generally summer classes are doable around work though.
I would be uneasy about is the winter class. It's very condensed, usually every day, multiple hours a day. Winter is a good time for travel/unwinding in my opinion. Maybe I would take 2 classes in teh summer and none in teh winter. But that's just up to you. It's personal preferance.
And finally, course pickings are extremely slim during the winter and less so but still slim during the summer. If you like having tons of classes to choose from its also better to do more during the semester.
Again I think the main reason people don't do what you're saying more often is the cost. And, sometimes people like to take extremely demanding internships or out of state internships during the summer.