I don't see how it would make turning the tide any different.  There's nothing stopping anyone from killing each other's mechs and vehicles.  And you still have to brake off and actually travel to the bay and pay for repairs, which is still far better than the bf or crysis method of hide behind a hill and do a 10 second repair,.  The only difference here is that allowing for the replacement parts means you don't have to buy new mechs every time your ride gets messed up.

Besides, assaults can't break away easily.  Everything is faster than them.  So if you want to make sure that the assaults that are kicking your ass don't come back shiny and new, you don't let them get away.  Bomb them with aircraft, riddle their rear armor, aim for the legs, swarm with elementals.  Go after the wounded.

Let me put it in these terms.  Imagine you are playing crysis, you just paid prestige points out the nose for an mbt.  But right off the bat, fresh out of the factory, somebody's lucky shot guts your treads.  Your driving now is all retarded and skewed.  Your tank is still alive, still has ammo and working guns.  But for whatever reason you can repair just about anything on the tank EXCEPT for the treads.  So now if you actually not want to be stuck in a death trap, you have to buy the same tank again.  Now as a player, do you think that would be A. Fun, or B. Frustrating as hell.

i'd vote for B

We're talking about arms, not treads.  And if you're defending a base in a last ditch effort, you're not going to chase an assault back to it's repair bay, no matter how much faster you are.

We're definitely coming from two perspectives on this, and I believe that in order to keep the flow of the game more unpredictable faster paced, arms shouldn't be replaced.  This will cause congestion in repair bays, yes, but the line at the repair bay will deter people and keep them in battle longer as well.