They're cheaper. *I guess if you don't need them very often a decent
powered one would not be cost effective.

Well not really.

Rock bottom from Screwfix is £29.99:

http://www.screwfix.com/prods/40588/...Mitre-Saw-240V

I think the budget supermarkets have gone even lower.

Compare that with the £18.85 for the most basic hand-operated
Axminster model.

I dare say cheaper can be found, but as the OP says, they suffer from
poor plastic guides.

There are also fine joinery applications where the hand-operated tool
can do things that are tricky with a powertool - but I imagine that
sales to people making fine joinery is a tiny fraction of overall
sales of hand-operated mitre saws.