It seems to me that every single time I sit through a training/presentation session on project management given by folks with PMP certifications they talk a lot about ways to get things delivered faster; Fast tracking, resource balancing, etc. Never have I had a PMP person talk about the difficulties in doing this and the reality that 1 resource does not equal another resource, there are tasks that cannot be done effectively by multiple resources and some tasks have specific time span constraints regardless of how many people are working on it.
For instance if a complex algorithm must be developed, once the logic is designed writing the code is likely not something that you want more than a couple of developers working on together. If you are dealing with testing a system and the test needs to take 3 days to execute because you need to let the system process for three days to exercise the system it might just take 1 person to push the start button and monitor the test. 3 people will not make it take 1 day. Not only that, more developers working on the same parts of a system will step on each other and actually slow down development.
I think it would be good for more PMPs to read The Mythical Man-Month: Essays on Software Engineering by Fred Brooks.