Consider this...Maps and Search were add ons to android, while Voice Dialer was installed on the G1 from day 1. I agree they do use different aspects of the libraries.
Ever notice that when in search you get a visual graph of your speech pattern after speaking while in Dialer you get the yellow spinner?
From what I have heard about how things work at Google:
Gmail team
Gmail for android application team
Gmail mobile team
Each are different teams of people with their own feature release schedules and the intent is to NOT be in sync. The reason that I was given...why hold up great features in one just to make sure the other team's ready. They each have their own challenges, roadmaps and fires to deal with and independently can make things better and more in tune with their team's needs. Obviously eventually a good feature or way of dealing with an issue might trickle over to the other teams. Cream rises to the top.
This is from inside Google, so I trust my source.