Despite the amount of firepower in such a fleet, it's not really optimal for attacking a single vessel as ten battleships would be overkill when a single battleship or battlecruiser would be able to do the same job. Really, the best choice would be a fleet of smaller, nimbler ships like Hina's hunter-killer squadron accompanied by a single large vessel. That way, the Navy doesn't have to commit ten battleships, which are fairly rare and probably very expensive to operate, to a long-term operation. The fact that the only battleships seen to date were Buster Call task force ships might well mean that the Navy retains them at the Headquarters and only dispatches them when necessary.
Of course, the Gorosei might eventually get pissed off enough at the Straw Hats to order the Navy into such an action whether it's economical or not.
The Buster Call is not effective in Open Waters, only in the confines of an island, because they can surround and do stationary bombardment of the island. In open waters, I can see it being fairly useless. If it was really that effective at deterring the high tier pirates in open waters, then the Yonkou (and the Shichibukai, for that matter) would be eliminated already.
Uh… he didn't say the most powerful Vice Admiral, but one of them.
My bad, I misread it.
Even then, there's no actual proof that Garp is the strongest Vice Admiral as we've never seen any of the other Vice Admirals besides Kuzan do anything besides give orders.
Yeah, I seriously doubt any of the other Vice Admirals could take a slash from a massive, razor-sharp axe and not wake up from their nap. Plus, it should be plainly obvious that Garp was at one time an Admiral; he either stepped down, semi-retired, or was demoted (Which wouldn't surprise me). Granted, that's speculation, but "The man who cornered Roger" wasn't just some run-of-the-mill marine.
Considering that the Buster Call is a military operation, yes, there may well be a need for one. If the World Government found out the location of the Revolutionary Army's headquarters, then they may well dispatch that much force in an attempt to annihilate that organization.
Like I said above, if it was truly as effective against a stationary target (Island) as it was on open waters, the Yonkou and the Shichibukai would be toast.