While I poke fun at Percy the giant squid mount (Noooooooo!), I don't really care one way or another about this rule. Out of all the new rules I would hazard the guess that this one is the easiest to house rule back to the Stone Age if that is your wish.
In the last campaign I was in my friend Eric played a paladin. In the first session his mount was injured by a polymorphed ogre. I will leave it to you, gentle reader, to puzzle out why a polymorphed ogre was anywhere near three 1st level PCs.
Anyway. Somewhere around 2nd level it was almost killed by a wolfwere. Finally it was eaten by owlbears. This was all before 3rd level if memory serves. I don't recall how number two bought the farm. The third one was given to him by a religious order. It died during a simultaneous attack by a swarm of stirges and an umber hulk, though the DM fudged like a madman to let the paladin save it.
It was also constantly being left behind. On one occasion it couldn't go down into a dungeon. No prob, if an umber hulk and its adopted owl bear and stirge parents don't eat it, we'll get it when we come out. But we didn't come out. Instead we had to ride the DM railroad express through a portal and ended up a few hundred miles away.
If Eric could have had that horse disappear for a while instead of being a meal on the hoof for the creeping unknown. I bet he'd have done it.
Hikaru, Scarbonac; I'm glad I could make you guys laugh. I had fun writing it.
In my campaign I have a guy that wrote up a whole back story for his PC who inherited a whistle from his dead father which when blown will summon a pegasus mount because he wants to be a paladin. *gasp*
We've all seen this.
Well, he understands he'll have to wait until 7th level and that's cool with him.
With the new system it actually works out really well. He blows the whistle, his mount can swoop in from over the horizon and aid him. When time runs out the pegasus leaves him eating dust because, let's face it, pegasi have better things to do; like mating with confused ponies and crapping on those really huge statues that exist in all fantasy worlds. Who are we kidding, do you know how many pigeons it would take?
It's been said before; whether or not this uses the mechanics of the summon spell, it doesn't have to be a spell. It's more like a summons to court, or to dine with the king.

It doesn't have to poof. Maybe it just shows up from around the corner
somehow.
Note: FYI, Eric's horse was essentially killed three times before he reached 10th level. If I want to beat that level of DM negligence (he just couldn't think of anything better to do) I'll have to kill that pegasus three times between 7th and 10th... Nah, I'll leave it alone.