Why on earth would you find a letter asking for referrals to be rude? That is how the better recruiters get candidates.
You are free to ignore. But you seem to want to know what is in it for you? Well taking the long view, if I recommend someone for a job I am not qualified for and he gets the job, then he owes me a favor. He also might be inclined to hire me because he knows my work just as I knew his, so it may be a way to get entry into an organization.
Further the recruiter knows you helped him. They will consider you a person they want to work with and where is the downside to that for you?
People who work with other people's needs and cooperate tend to get more cooperation in the long run. People who want to be given something up front before they will make any effort no matter how minor (usually when I recommend someone to a recruiter, it takes me less than five mintues, maybe ten if I don't have their contact information handy.) come across as arrogant and greedy and hard to work with. Is that really the impression you want people, especially those who have say in what potential future opportunities come your way, to have of you?
Asking for a referral bonus from a recruiter would be rude and inappropriate. It would pretty much guarantee he won't want to work with you in the future or use the person you recommended. That is a lose-lose situation, through greed you lose a possibly value future contact, your recommendee loses out on a job he might be interested in and nobody wins. And really how much work did you do? You wrote an email with a name and contact information. Asking for compensation for that just looks petty.