Here's where I can see melee weapons being effective. Suppose there's a HoF that's causing a lot of trouble and you need to get the flag because, well, its capture the flag. (I play CTF on legions, so most of my scenarios come from CTF.) The team, provided this is not a PUB, and teamwork is being used, can send in two people along the same cap route. The first "capper", however, melees the HoF aside and the second one comes through and takes the flag.
Also, there could be different melee weapons for each armor type. For the sentinels, there would be a melee weapon that isn't a KE charged weapon, but have a range of decreasing effect. Basically, the closer you are the the Sentinel's melee weapon, the more damage is taken and the more it slows you down. The slowing effect could be partially mitigated if the target is in OD.
The raider's melee weapon would be KE charged and have a short (.5-1 meter. Keep in mind that legionnaires are 2-3 m tall) and just deliver a destructive blow like a shotgun, but with energy rather than shot. Also, instead of a decreasing effect over range, it would just have the same amount of damage delivered no matter where inside the range the target is.
The outrider would have a KE charged lance that would only deal damage if the point hit, but do less damage than the other two's. However, it would, upon dealing damage, deliver a knockback with the rest of the kinetic energy charged. (This is incredibly fake science, but hey, its a game) This, if used correctly, could be used by a capper to clear the flag of bodyblockers by themselves if they aren't comfortable, or able to make an effective OD (ie: their health is low enough that they could get away, but if they OD, they will learn something about it.)
These are my thoughts about melee weapons thus far.