Here's how I think of it.
Going to a trail centre - a good one* is like a track day, for a bike - where in this metaphor the PROW network, is the roads. When on those, you must be sensible, show consideration to other, obey some rule and generally (though not always - not hoon about too much).
Trail centres are where you can hoon, pin, etc - you don't have to slow up for doddery walkers, there aren't gates and fields. You can ride it fast and assume that anyone round the corner will be on the look out for you, and step off the trail**. Its a place to go for thrills, or maybe just because if well made, they're weather proof and you can ride when all else has turned to slop. Living where I live I would hate not to be able to ride custom made trails.
*my definition used to be a trail centre that is either in Scotland, or has NO CAFE/FACILITES - eg Machno, Stainburn, Gisburn etc. Those in the know will have spotted my problem. Gisburn is about to get a cafe... this is disappointing. I should, really, strike it firmly from my list. But I like it. So we shall see what happens next time I go there. Will it feel there same? I might just pretend the VC doesn't exist and still park at Tosside. I understand the point above about depressing. Dalby VC carpark, with its procession of people numbly spinning out of the carpark and into the woods on the zig zag line, takes a wonderful and liberating hobby, and makes it feel like going round Ikea. This is why I feel very privileged to live 20 minutes from Stainburn. From the car park there are 6 directions you can head in. It feels more spontaneous, and free.
**yes you should ride within limits and be able to stop, but that's different to it being likely you will need to stop - an emergency stop can be ungraceful on a trail where on a PROW it shouldn't really be threatening/unnerving to other users.