Originally Posted by
HillRider
Incidentally, Shimano never produced compact-specific front derailleurs saying they were unnecessary. After a couple of years, Campy agreed and dropped them from their product line.
Necessary is a matter of opinion which can change very quickly depending on the market and cost of producing more permutations of a product. Every tooth changes the radius of a chainring by about 1/12" so a 50t ring would come up about 1/4" lower than a 53t. Depending on the height of the braze on and length of it's slot, this can make for FD locating issues when changing rings.
There's also the issue of the curvature of the cage, Going to smaller rings is rarely a problem, but going to rings larger than those the FD is designed for can cause the heel to touch the ring while the front is well above the 2mm clearance target.
So while 50 or 53t may no be a problem, bigger changes, or using mtb FD with 53t rings can be.