Good point! Reserving DPs makes the later development feel more natural, though. It's more like finding out you're good at something you never tried before. Also, players don't get better than they usually would, they just get better at the exactly right moment in the exactly right skill.
Fate Point development feels more heroic, and messes less with game mechanics, which is a big plus. Then again, it is a pretty big bonus for any one fate point (because this could really accelerates skill development more than would be available otherwise).
Somehow, the original idea sounds "more right" to me, but I can't really say why. Your cost-argument makes total sense, but somehow being able to buy skill ranks with FPs seems to cheap (because it can so accelerate character development - I might be especially sensitive, because in our group, we don't buy FPs, but earn them through roleplaying. Feels like this mixes different currencies (RP and char dev)).
Don't really know.
In my group, I'll propose reserving a maximum of 4 DPs (sorry for the "5 DPs"-option, that doesn't really make sense because of the 2/ 4 costs), and they can spend them whenever they feel like it (but only on skills they don't have - otherwise, they can spend them usually with the next level-up).