Wikipedia:I don't see why not

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This page in a nutshell: Whenever there's a proposal, I don't see why not is a bad reason while sometimes I don't see why might work.

Whenever there is a proposal, there is bound to be someone who says, Support. I don't see why not. Others will say Oppose. I don't see why. While both may not be reasons themselves, a lot of users use them as a kind of argument. But are they appropriate?