For creative freedom.

Stephen King released some books under a different name because he had issues with people buying his books just because they were his (and because he felt closed in with the "Stephen King" stigma).

Gilmour has said a number of times that a certain product is expected when you create a Pink Floyd album...so if they DID feel like recording together again, but didn't want all the expectations...using a different name would be a good way to do it.

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Only the very safe,
Can talk about wrong and right.
Of those that are forced to choose,
There's some who will choose to fight.