(with ios9 , it's only a bigger ipad with a magnificent pencil, but again, only a BIG iPad.
this thinking is contradictory, because it is the new addition of the stylus which means the pro is not simply "a big iPad". Personally I think it would be an extravagant waste of money if it were just a bigger iPad, but the stylus brings a whole new dimension if you are are artist, designer, its new territory for apple and at last they've created a stylus / tablet.
It's less comfortable to used with ipad tasks as reading magazines, newspapers, books, watching movies...
it may be slightly heavier, but these things you mention will be MUCH better on the pro - think about it, magazine, paper, comics at a real life size, much easier and nicer to read. If you look at the original iPad / air screen size objectively, it is a bit cramped when rendering a magazine or comic, to me this looks like an advantage for the pro not a disadvantage.
Also regarding holding - for prolonged things like a movie you would usually not hold the iPad, and holding even an air for 2 hours would not be comfortable, you usually stand it on a desk for a movie or tv, or have it resting on your lap partly or on your stomach in bed.
And it's less useful to used with pro tasks(editing office documents, dwg files, decodind video, using photoshop)...
it depends on your profession and interests. I mean for artists and designers this is going to be much more capable and intuitive a creative tool than one of those old tablets you plug into your mac or pc like a track pad, and have to draw down on the table while it appears on the screen in front, I hated that and could never get used to it.
I could give you more reasons, but think about this: It's a niche product, only for very specific people. Someday, ipads and macbooks will converge in one product( yes, Tim!!!, you know that it' true) and it will be THE PRODUCT. Until that, it's only one step more to that way, but this time, a little step.