To begin with: Summoning a high force spirit is a last-ditch thing. You don't do it when the HTR team pulls up (you have much better options at that stage) - you do it when things are looking like they're going against you. A significant percentage of the time, it will KILL YOU. People don't go around doing stuff that's got a high chance of killing them - and it's not at all rational behaviour. And banishing isn't the only countermeasure against spirits; in point of fact, an Ares Alpha with APDS loaded will hurt anything up to Force 18 on a single net hit. Even just a Predator with APDS will be able to tag any spirit Force 12 or lower.
As for cover: The Street Sam can and should be sticking to cover just as much as the mage - he can fire from cover.
As for Enchanting Mages and Riggers, well, Riggers can reasonably expect to have their vehicles with them and such - and can pretty much expect to have their drones with them almost any time they might need them. Alchemy takes minutes. That's a lot different from needing a set minimum of materials (that may not be easy to get ahold of, depending on circumstance) and a matter of hours. You keep bound spirits around, yes, but you have absolutely no flexibility in that arrangement - its fair for a conjurer to have less flexibility, but not to have NONE.
As for the spirit power dynamics - it's a long standing thing, and a good way to deal with spirit power issues (either you have serious issues summoning powerful spirits, or you're being careful with what you're asking them to do, like actually using your spirits for the things they're associated with and not having them take stupid risks for you).