Mark Rippetoe, (of 5x5 fame, as well as being a genius strength coach) has an interesting thing to say on 5x5 vs. pyramiding.
He says (and I'm totally paraphrasing off the top of my head as I can't find the exact passage in his book):
"When you cut out all the extra bull, getting stronger means being able to move more weight than you could before. Training with low reps (1-5) works the muscle very differently than pyramid training where you eventually end up using a lot of reps.
The problem with pyramiding is that while you end up exhausting the muscle, by the time you get to your heavy weight the muscle is exhausted and the potential to move up to a higher, previously unatainable weight is greatly deminished. You may have what some lifters refer to as a pump in your muscles, but that's just blood flow and bloodflow in and of itself doesn't build muscle or make you stronger."
I'm sure he put it much better than I did, but the basic meaning is that while pyramiding is great for muscular endurance, it's not so great for building strength. If a muscle is ever going to grow, it has to get stronger.