Hello!
Peter: Nobody in GA "needs" FBOs.
Don't confuse GA with GA :-)
There are GA pilots who fly privately, they may or may not need handling services.
And then there are those GA pilots who fly commercially, either as corporate pilots or within a public transport operation (AOC). We (the latter ones) need handling! Otherwise we will not be able to provide the level of service and care to our passengers that they pay for. You simply cannot walk a group of "VIP"s over the apron (after having dressed them in hi-viz-jackets when in the UK...) and drop them in front of some airport building in the middle of nowhere.
We also need a place to stay warm and comfortable while waiting for our - most often delayed - passengers. Sometimes the duty time regulations call for crew rest facilities. Food and drink for crews, catering for the passengers on the return trip, filing and delaying of flight plans, ground power for some aircraft - all these are essential services provided by handling agents.
Ljubljana and Portoroz (Slovenia)
Besides these two extremely friendly ones I can add Graz which is also in vicinity.
Don't confuse GA with GA :-)
I know... there is a demand for comfortable facilities.
The problem is that at most "mandatory handling" places one doesn't get that much of that type of traffic. Where I am based, you get a PC12 or KA turn up, with passengers who have just spent €10,000 and expect appropriate facilities, a few times a year...
But the management seizes on this opportunity to make handling mandatory.
The argument is that if they didn't make it mandatory, it would not be viable. Which I am sure is true 
Exactly this, with exactly this argument, is what has brought mandatory >2500kg handling to my local.
Unfortunately, this, in turn, has practically killed off 2500kg+ visitors - other than ones going for maintenance who are exempt. 2.5T+ owners (say, a 421C) are used to paying quite a bit everywhere they go but once the landing fee goes into 3 digits they start to think twice about it.
For turboprop/jet owners, it would probably be true that they are paying such a lot to fly (€500/hr+ baseline for a TBM) that they couldn't care less about a €200 landing charge, but from what I see very few of those are doing casual trips on a nice sunny day.
So the result is that the airport loses a lot of business.
I also know a couple of pilots of KAs and bizjets who say that, suprisingly, a good % of their clients do not want the €500 handling experience. They would rather keep the money.
