Hillgate - that's simply not true. Look for instance at Hilary Johnson's website; her
author's list.
Many of these are well-known names, some of them prize-winners, bestsellers - Katy Fforde, for instance, and Christina Jones. These are all writers who used an assessment agency. It really is a question of finding the right one. Me, I'm glad I used one. The £300 fee was worth every penny.
It seems to me that in the UK, whereas it;s not true that "most" agents use these agencies as scouts, the reverse IS often true - that authors who realise that they are nearly there use them for that last polish - and for the referral, as a fast track option.
I'm a member of WriteWords, a major UK writers' discussion board, and I know that many of best writers there have used them, and recommendations are discussed openly on the forum.
So it's not so much the no-hope writers, as the talented ones you might find seeking these services. Which helps improve their reputation, which in tern lures in more good writers.
In the case of Hilary, she only works with one agent as far as I know.