Glen Davis didn't actually contribute that much to the 2008 playoff run. He played a mere 137 minutes across 17 games in those playoffs. He emerged as more of a rotation member in the subsequent seasons.
Eddie House is also being misremembered. He played only a little bit more than Big Baby in those playoffs, just 167 minutes. He also became more prominent in the subsequent seasons.
In that 2008 playoff run, the real important bench players were Posey, PJ Brown, Leon Powe and Sam Cassell. Those guys got a ton more minutes and were simply much more critical. Sam played the least of them and even he got 264 minutes. Powe, of course, had a key performance during the Finals that will always be the signature of his oh-too-short career.
People tend to remember Big Baby and Eddie because they both played larger roles with the team in the following years. But neither was nearly as important as those other 4 guys to the 2008 playoff run.