I agree with mf1 above. I have frequently liased very successfully with clients' solicitors with a view to easing the process for the client in general. It's no fun when a lender's call centre merrily tells a client or their solicitor that "Documents X, Y & Z are outstanding" when the client / solicitor / broker hand-delivered all three a couple of days ago.
If there's a broker in the middle, the client or solicitor rings the broker. The broker then gets back on to the lender's call centre, sits in a hold queue for ten minutes, then gets passed around various muppets until eventually locating someone who can confirm that the aforementioned documents are in fact in their office. A mortgage broker or conveyancing solicitor will be familiar with this and similar tales - they occur on a daily basis.
I'm not for a moment complaining about the lot of the mortgage broker. Just pointing out that a good mortgage broker will do a fair amount more than just taking an application from a client and posting it on to the lender. If people deal directly with lenders, they will take on more work themselves or may attempt to pass it on to their solicitor. Fun for all concerned.
(For those not already aware, I am a broker, although not exclusively a mortgage broker.)