There are two points to having other types of units. The first is how long a unit takes to build. All of the types you named are expensive to build which takes longer. A garrison unit on the other hand costs less than half to build both in time and materials. If you need units quickly then you can get a bunch of garrison units built compared to the marines or armored units. The defense value is the only one that is used to pacify a population so that gives you comparativly more value per cost.
The second point is a little more nebulous as I can't find the reference, but I think that the maintenance cost is based on the build cost. More expensive units cost more to maintain. If you need lots of troops spread around then the cheaper units will cost less.
For actual combat ability, ie an invasion where you need the most firepower for what you transport, the heavy units are far preferable as they will give more combat power per unit than anything else. With the recent changes you might even want to bring along a combat engineer or two for dealing with pdc's. I think they would be best in followup landings and not part of the initial assault where they are comparativly weaker.
Brian
