Poof
How many literary knights and paladins had their mounts pop off after ten hours, with no recourse to get them back for the rest of the day?
I'm all well and good with a paladin being able to "whistle up" his horse. What I'm not fine with is the fact that, no matter what anyone says, after eight, or ten, or however many hours, the horse "poofs" off - or however one wants to describe it. There is absolutely no getting around the fact that the horse will not always be by the paladins side when he needs or wants it. It's great and all that after the teleportation spell is popped off, the paladin can trust in the fact that when he goes from Bumpkissville to Podunk Town, Beetlebaum the Warhorse will be there waiting for him. What's not so hot is that the steed becomes a spell, not a companion. Unless the paladin is level 12, the horse will not always be there. Sure, it means if the horse gets beaten into tar, that when watches come around, it won't be there to be killed at the last, but the fact that it's at risk is, in part, why it would be cared about - the fact that it is, ultimately, mortal, and doesn't just trot on up to heaven halfway through the day.
Giving it a duration makes it much, much more like a spell, and much, much less of a companion. Companions shouldn't have a duration. This seems to be the one thing being neglected in the mention of Bill, or Shadowfax - the fact that they didn't disappear after a certain amount of time, because, despite being there when needed, almost no matter what the circumstaances, they were still very real flesh and blood creatures that required stabling, feeding, and didn't just fold up into some extra-dimensional space.
Dramatic entrances are fine; spell-like durations are not.