As a practising solicitor, I made a decision about 5 years ago that my firm would not purchase sites encumbered by such a condition, nor would we be party to a sale of them. (and no, we weren't buying lots of them before that - such conditions were once very rare). However, at least once a year I will have a client looking to buy or sell such a site. Although I always refuse to act, they always find a solicitor prepared to deal with it, as indeed does the person on the other side of the transaction.
The reality is that these planning conditions do not seem to be in any way policed; there is also, apparently, some small doubt about their legality\enforceability and this seems to be enough for many solicitors to feel happy to ignore them. I wish it were otherwise, but that is the reality.