I think it's fine within reason. It shouldn't be, 11 people in a group are each sitting at a different 4-person table, waiting on the one person in line. But if they're occupying 3 adjacent 4-person tables, I don't see a problem. I would rather have them sitting in the restaurant than standing in line. If they're organized, so the one person ordering can say, "I want 6 hamburgers and 6 cheeseburgers, 8 orders of fries..." that actually goes faster for everyone else, than 12 individuals placing and paying for their own orders.
And as someone else said, I want to know that I have a place to sit before I start placing my order. Because I could get it to go or even go someplace else entirely, if the place is too crowded.
Personally, I do think it's better to leave a person at a table to reserve it, than to leave a thing like a coat or umbrella. Because if it's crowded, the person can tell you how many seats they're saving (especially at places where a lot of the tables are connected). And there's always the possibility that the coat or umbrella was just forgotten, unnecessarily removing that whole table from the pool.
Of course if a restaurant doesn't like this, or they find it doesn't work for their process, they are free to post rules to the contrary, and people should abide by them.