is this a common "problem" with FXs? I am using word "problem" loosely
Yes.
As you point out yourself though, it is not a problem so much as it is simply something that happens over time. Every car suffers from this to some degree, the TB will require cleaning. It is a simple process that any dealer will perform when they change your intake filter, or during the course of routine maintenance, they spray a little TB cleaner in there and it will clear out any deposits.
Again Cruisers TB does look dirtier than most normally would, this could be cause by a number of things. If cruiser could answer a couple of questions we can determine if this is a problem worth pursuing further or if it is something to just keep an eye on for now.
Cruiser:
1. When was your TB cleaned last to your knowledge?
2. What grade fuel do you use in your car?
3. When was your Air filter last replaced?
4. When did you last change your spark plugs?
From the looks above, it seems as though it had simply been a long time since the TB was cleaned, but it could be that he uses lower grade fuels and the combustion of this fuel is leaving more deposits.
If neither of these are true a bad plug could be causing an excess of build up. (this is less likely because his plenum looked pretty clean and for this to be the case I would expect the plenum to be very dirty as well).
Overall even the substantial deposits seen above are not a problem, cleaning the TB will return the engine to operating perfectly, and it has not done damage to any other components to other parts. Having a dirty TB is nothing like having dirty oil in your car, it will not cause damage to other pieces. The FX willl however idle a little more rough, and start-up may leave a little to be desired.