The Invisible walls are only in spots where people will get needlessly stuck (sorry, there's NOTHING in the ocean for you or that deep crack in the planet so no point in being there) or if they want players to generally follow the level design. Remember, all throughout Beta and early 2.0 days people complained endlessly about that small jump at Camp Bentbranch where people would get stuck if they tried to take a small shortcut...something invisible walls would have completely fixed.

Despite popular belief, it is not due to consoles, because I've played plenty of PC ONLY MMORPGs that has them because not everything is meant for player interaction - some places you 'can't explore' doesn't even have geometry you can stand on, it's there for show most of the time.