Life would be simpler if the professionals involved took the time to understand the PD regs and guidance, and then put their neck on the line and gave out firm advice to their clients.
The regs and guidance are however quite complicated in some cases, and in my experience, many professionals cannot be bothered / don't have the time to gain the level of understanding required to give out firm advice.
Some probably do give out that advice, but the client still wants a LDC for peace of mind or to have some formal paperwork to hand over when they sell. Probably a good idea, as many of the professionals I deal with apply for LDCs, for things which are not PD, with alarming regularity.
I would think that most solicitors, whilst they will be familiar with the concept of PD, do not have the detailed expertise required to ascertain whether many developments are actually PD.
Some things are simpler (e.g. a rooflight or replacement windows on a single dwelling), other things are much more complicated- e.g. I have just seen another thread where a council has granted prior approval or said it is not required, but the extension is not PD and in fact needs planning permission. So if that client followed up their prior approval application with a LDC application, that should be refused. If they have built that extension, it is an unauthorised development.
It's that sort of complicated scenario, which is a product of the complexity / poor wording of the regs and guidance, which it takes quite a lot of research and experience to get one's head around.
If the regs were simpler, the professionals and clients involved would be much more likely to proceed without a LDC. The more complicated the regs are, the more likely those involved will be to resort to an LDC.
Personally I would advocate simpler regs, even if that meant a few less thing were PD as a result..