I think it depends on how many are in the group and the size of portions the restaurant serves.
If, say, you absolutely love the little lamb kebabs that come with 3 on a plate with a special sauce but a couple of others love the meatballs that come with 4 or 5 in a dish, so they get a couple of dishes - then if there are several of you at the meal, there's a high chance that you will get only 1 or none of your little lamb kebabs that you are really fond of, whereas the people that like meatballs will get a couple each plus there are some extras for others to have one each to try, so they can't see a problem with having one of your kebabs if you are having one of their meatballs. Can all end up very out of sync if you are the person whose favourite dish disappears before you get to it and all you end up with is a meatball or two that nobody else wanted when you don't even like meatballs...
In which case - I think it is reasonable to say that you want to keep your lamb kebabs for yourself as you don't like meatballs, if others want some to try then they can order some for themselves. If you end up in a group where there are some people that are greedy or selfish you can find yourself having a very unsatisfying meal whilst they think that it's marvellous because they've taken all the really nice stuff and nobody else gets a look in.
On the other hand, things like your patas bravas and tortilla that everybody wants a scoop or slice of as a basic part of the meal, then it is sensible to order xx of each for the table, see how it goes down and then decide whether or not the table needs more of either of them. So ordering these as a table makes much more sense as chances are you're not going to want to eat a whole portion of both but starting off with some and then adding more as needed (and when you can see which you like the best and fancy some more of) makes good sense, plus means you don't end up with lots of uneaten food.
I used to go to a lovely spanish restaurant with friends. Going with one or two friends, we were able to order between ourselves and would share the food, it was ordered on that basis and we knew what we liked and that we would both get enough of the bits we liked. However in bigger groups - we would definitely order one or two things each along with some sharing things. Then you could have your fill of the bits you wanted and if there was anything left over you could open it up for others to have - or swap something with a neighbour as you wanted, that way everyone is happy.
Sorry it's not a simple answer!
However I do agree that it's plain odd to take part in a discussion about getting a good mix and then say that their food is not up for grabs - are they expecting to be able to grab at other's food though?