There are pros and cons. Depends on the individual business.
You need hold no stock.
You don't need cutting equipment.
You have no control over the cut quality or picking and choosing parts of a moulding. Stuff like oak/ash varies in grain pattern and it's nice to match rails up. Same with knotty pine.
The time saved cutting is just about cancelled by the time taken ordering.
If you have a ubiquitous 'magic' moulding that you use all the time, it makes more sense to cut your own. However, now and then you might need a very pricey moulding that you may never use again. Chop is a bit more attractive in this case. If you bought the moulding and cut it yourself, you might need to buy 20ft and have 8ft left over. On paper, the 8ft may be worth £50. In practice it is scrap. You'll stick it in the corner and move it about for the next 20 years. In this case chop may work out cheaper.
You can't spend moulding at Tescos.
I only use chop for aluminium mouldings.