No reason whatsoever, although I think it would be a touch of overkill. Think about it for a moment: once you have the limit screws set properly, how often do you touch them again? At which oint, does it really matter what heads are used?
As you can probably guess, the manufacturers use philips and straight head screws for cost. Probably saves them $00.005 per derailleur - which adds up over a year's production.
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Syke
“No one in this world, so far as I know — and I have searched the records for years, and employed agents to help me — has ever lost money by underestimating the intelligence of the great masses of the plain people. Nor has anyone ever lost public office thereby.”
H.L. Mencken, (1926)