I'm sure it's possible to ride Pentlands, trail centres etc on a cx bike, I've just not found it much fun.
I find it MORE fun!
The Pentlands in particular could have been built for CX bikes and they're capable of most things that the average "XC MTB" can do – sure in some situations they'll be slower or a bit more twitchy but there's plenty of terrain where they'll be at least as fast if not faster.
They make great sense in winter especially, they'll just cut through the mud that would be wearing out all the pivots on a full sus, it improves your handling and riding skills and it's just a bit different. Even a well-used route can become new and interesting the first time you try it on a CX. They're incredibly resilient and versatile bikes – thrash it senseless for an hour round a race, load it up for a weeks touring, you can do the local roadie chaingang on it, pootle along some towpaths, whatever.
They've never been designed to compete with MTBs; they'll never win a DH race or anything but they are true jack-of-all-trades bikes.