For a well known example of how this matters, consider the Pentium 4. Intel designed it with a deep pipeline on the assumption that they'd be able to reach higher clock speeds than was actually possible. At the intended 10GHz, the Pentium 4 would have had impressive performance for the time. But they couldn't achieve that clock speed, so they had to revert to the simpler pipeline design first used in the Pentium Pro and continue development from there.