I agree it is a valuable lesson Martin needs to learn
But in the case of Elaine, I think it was just a miss by the writers. The character didn't work and the shtick grew tiresome fast.
I think that's why they went in a different direction in Series 2.
But perhaps there Is a behind-the-scenes story to the change in receptionists??
I believe Lucy Punch is making it big in the US...maybe she had a better offer on the table after Series 1 of DM...?