IMHO the time-skip was completely unnecessary.
The time-skip happened so the main characters could get stronger. But their new power was not necessary, since the villains are as strong as the story required.
If the time-skip didn't happened, Luffy would have defeated Hody Jones and other villains the exact same way, since we did not have a reference of their strength before.
The only time it was necessary was against the two Pacifistas in Shabondy, but they only appeared in one chapter and were never seen again. If you cut that, there's zero need for the power-ups.
In anything, the massive jump in strength during the time-skip only create plot-holes. It's hard to believe Luffy was the center of attention on Marineford, when there were so many enemies 100 times stronger than him. Why were the Pacifistas such a big deal, if Whitebeard has a few commanders who could defeated all of them with little effort?
Everything is about Haki now, and Luffy could have learned it in a few days instead. It would make more sense than Luffy training for 2 years, but only knowing the basics and mastering important techniques out of nowhere.
Without the time-skip, the story would only require a few minor changes. Instead of hearing about how Law, Akainu and Blackbeard got their new positions as something of the past, it would happen concurrently with the story. But that would totally sucks, right??