Another reason many authors don't want to recommend to their agents/editors is because your submission becomes a reflection on THEM, like it or not.
So, if they look and this friend of their client is fantastic, then that's a positive reflection. If the books sucks, or the presentation is bad, or your attitude just hits them as wrong, that's a reflection on the person who did the recommendation.
On the flip side, if the agent doesn't take you on, it could sour your friend's view of them.
I know of at least one writer/agent relationship that went seriously south and ended after the writer recommended a friend's work that she fell in love with. The agent rejected with a form letter and the writer took insult on her friend's behalf. Sad state of affairs, but it is human nature.