Are you talking about deep cracks that go all the way through or just many very small cracks? These boots look sort of cracked with these many surface cracks even when the cars are relatively new. You're doing the right thing by keeping an eye on them and watching for leaks. When they start leaking take action soon, 'til then I would just keep an eye on them.
My 240D's boots have had the cracked look for years and have never been replaced, leaked or given trouble.