The disadvantage of a glossy screen is that at some angles, you'll get more glare. The advantage is that (as you note) if you modify its angle, you get less glare than you would with a matte surface.

Agreed that you sometimes need those extra hours. But to be fair you have to count their weight as well; you can double the usable life of a MacBook Air with an external battery if you need to but it also doubles the weight.