"mechs just aren't good combat units in general". True, but they can provide good fire support for mbt's in some situations(remember C&C3 where Mammoth tanks were leading attack, taking most damage while Steel Talons medium mechs used their heigth to shoot over them, keeping themself safe behind heavy armor and providing additional firepower?)
But the height of their guns has nothing to do with having legs, they're simply taller (you can make a tall tank). Mechs are inefficient by weight and are overly complex, so their only use lies in doing what a tank absolutely cannot do. Examples would be traversing mountain terrain and other steep inclines of soft ground, or highly uneven rubble.
Can you imagine how a tall tank will look like?
Halo's Scorpion tank 
and here we are talking about mech's "Rule of Cool" "because it's not trying to be realistic.":\
I wanted to say something else, but I forgot...
I wanted to say something else, but I forgot...