That would be nonsensical. There's nothing a mobile unit could do that a PDC can't.
What do you mean? If I have a missile on a trailer designed to look like a Coke truck, eliminating it from long range is a bit more difficult. And once I fire, it's just a truck. Admittedly, I generally deal with PDCs from orbit, so I'm assuming that they're gone, and you don't want the enemy bombarding you unhindered. Also, it would kill a certain percentage of landing troops unless suppressed.
Light infantry is generally used that way today. The problem is that in Aurora it doesn't work that way. At the moment, any sort of special mission is best undertaken by heavy assault troops, as they have the same transport requirements and more firepower. An alternative is to impose some sort of unloading time penalty on units that aren't infantry (light infantry and marines). However, unloading transports can't be shot at if they're at a friendly colony with sufficient troops. I'm not sure how it would work, but it's the only way I see to allow light infantry to do the job described. If we get some sort of map (which would be cool, but I'm not going to hold my breath) then Market Garden-type operations become more plausible.
What I see them doing under my outline (discounting the above suggestion) is working as either fleet troops (Marines, but without the boarding training) or as garrison units. Mobility is no longer a factor, and lack of armor can be dealt with by digging in.
Under the combined arms plan, it might make sense to only allow infantry units to board, and hindering combat by any other units caught aboard transports. Clearing PDCs would be another nice role for them.