This is in the PvP world. Lee - He means make the fence taller, and then have like a 4 block opening, so it isn't as easy to run away and out of the PvP arena, but possible to go in and out.
Even if this was implemented, people who choose to camp the border would do so on the inside of the fence, if this was implemented you would have to make the border a full block to prevent it from continuing.
This can be both good and bad. Good in that once you're in PvP arena, it's harder to get out. The openings will only be 4 blocks wide each, with 4 openings only. However, this only encourages more camping at the 4 block entrance since nobody would want to risk being pushed out into the arena, and everybody would be cluttered. You would also have to go back to spawn or walk a long way just to choose a different PvP entrance area. So I think overall, just keep it as it is, otherwise there will be even more annoying spawn campers. With a jumpable fence, more people will be obliged to fight, stimulating more action in the arena.
You could always make it so parts of the fence cannot be jumped over and some parts can be passed through, so there are the 4 main entrances and then a a dozen smaller escape ways
