Not that uncommon in commercial, at least in hospitals. We've got quite a few linear diffusers set up like this in waiting rooms, and some procedure rooms. Balancing is harder, but the comfort level is much nicer (especially in a waiting room, where everyone is just sitting around) than traditional registers would be.
The downside I see to this is the window washing also means it is directly disrupting curtains. I also, don't see the point, in the whole window washing idea. If the windows are the area of most heat gain, I wouldn't necessarily want to increase the temp differential across them by blowing the coldest air right on them. All this does is increase overall heat gain, for a minimal increase in comfort for 6 inches in front of the window (not usually an occupied region).
The buildings that I maintain have linear diffusers thruout them. They would be very effective, except for the fact that they are 4' back from the windows and there are desks where people sit against the windows. So the people that sit next to the windows complain about the cold air blowing on them, or the people away from the windows complain about the temp. Then there is the fact that the designer decided that the linear diffuser should also be used for the interior vents, instead of a 4-way ceiling grille. So they are nice for the right application.