Does it feel like the sole of your shoe flexes more when you pedal with them? If yes, and you like a stiff shoe, then maybe you want to replace them.

I'm not certain how catastrophic it is if the integrity of your shoe sole fails when you're on the bike - but I really can't imagine it being all that bad - so I don't see a problem with using the shoe until it actually does fail if it's integrity has been compromised.