7 Hours of play through in and I have noticed that a fighters ability to engage 5 targets at once, is an ability that makes it your best front line character hands down. It stops Mobs from walking around the fighter like they would if you had a paladin or a monk. 

Ya, but really you need two tanks, and a Paladin built in a medium way can do both and be your second tank that swaps to a two handed sword or something to damage when not needing to tank.

Kinda, but in my experience not really. Or rather, with by the time the third enemy engages the fighter and the rest have to go around, they end naturally passing far enough away that they don't trigger engagement even if the fighter is not at the engagement limit. I'd go for at least one extra tank to create something of a "wall", and maybe an dps/tank hybrid to deal with outflankers when the enemy group is still big enough and you lack tight corridors to avoid such (which is also kinda the only somewhat useful uses for a ranger's pet that I found other than occasionally flanking very high deflection enemies )