Hi,
While you'd probably get a better answer, with more references, on the DBA fanaticus forum, I'll try to give my understanding here.
The fast troop Type is an evolution that has been imported from the DBM/MM rules sets, which are the big brothers of DBA. The type is trying to represent the troops that fought in a looser formation than the 'solid' type allows for. It is to add a little bit more depth to the troop types, without adding too much extra complexity.
So in your example, the fast pike are actually modeled an Auxiliary (X) in DBM/MM. The type of army that has them are the early Scots or free canton medieval Germans. These armies fought with long spears/pikes in a similar, but much looser, formation compared to a Macedonian phalanx. Fast foot are more likely to recoil from opponents and they cannot provide mutual support to friendly elements in certain situations.
There are some oddities in the rules but I don't mind them and think these are expected with a rules set covering 4500yrs of history.
Hope that goes some way to answering your question!
LS
