I think that about sums it up, except to give consideration to how vehicles can compliment each of these roles. But it makes sense. You have big, slow heavy vehicles that do a lot of damage, and you have fast movers that do medium amounts of damage.
It stands to reason tanks lend themselves to both heavy offensive and defensive duties, bikes are awesome getaway vehicles for cappers and shrikes lend themselves well to the chaser's role.
Oh, and you can spawn vehicles in the free roam practice modes to hone your piloting skills and find new and interesting places to park your tank.