The word "unnecessary" is there for a reason, although perhaps it wasn't the best choice either. Of course you will always have abstractions, but the original abstraction of a saving throw didn't do a very good of representing a lot of situations that come up in dungeon crawling. They furthermore aren't interactive on the defender's part, and cancel magical effects without any explanation of what is going on. Hit points are an abstractions as well, but you at least have an idea that when someone has taken damage, that they are in some way hurt. If they took damage from a fireball, they should be somewhat burnt. If it was a sword, they are cut up. Saving throws just stop the gaming fiction on its tracks and don't mesh well with any kind of player idea of counter-action (you can, of course, allow the players to do all the things I mentioned above in a Pathfinder game if you want. But the saving throw system is not helping you do it).
Edit: As for this slowing down the game, I don't think this is significantly more complex than the stuff that Shadowrun 3e did. The GM should be ready to make a ruling on anything too crazy the PCs come up with, and the system can help with more common situations (for instance, how far can you throw yourself from your current position away from a fireball blast, given speed and whatever other abilities that might affect this).
