I don't think the curved back looks out of fashion/date, I really like it, plus bare in mind that aluminium is a different material that adds a classy vintage feel to the ipad, that the curved back iphones lacked. As you said it also enables the use of a larger battery and I would add better handling of the device as well.
Think about it this way, if an ipad lies flat on a surface, how are you going to pick it up if there are no curved edges? You would have to drag it to the end of the table, bench etc. to get a grip on it, or even worse use two hands. But now with the curved design you can actually pick it up from the curved edge that leaves room for your fingers to slide under. The iphone on the other hand can be picked up with one hand by it's edges.
As with most things apple, it's very well thought out and cleverly designed.