I have no interest in the PvE aspect of the game, unless you count killing city bosses (on a PvP server) and dozens of opposing faction players as a PvE event.

10-Boxing opens 10-man Rated Battlegrounds.
The same as it opens 10-man Raid Content.

10-Boxing is a lot more power, in PvP.
It is a deterrent against being ganked, or someone making the attempt... more so than five.

Obviously 10-box is for 40-man battlegrounds or opposing cities; don't think you could do that well in 15-man BG's but might do well in WSG (or anywhere else where there is only one objective to guard).

Ultimately, it comes down to what you find fun.