Just do it. I am a registration fanatic, I feel a horse's papers are his 'birth certificate' or 'passport' and don't believe they should be deprived of their heritage if it's available to them.
A registered horse can show at open and breed shows, is sometimes eligible for various prize and money programs not available to an unregistered horse. Sometimes, even if all you do is trail ride, the registry has prize and recognition programs for time spent in the saddle for its registered members.
Papers make your horse more saleable in some cases. If you decided to sell your horse but put that he was registerable but not registered, people like me will skip right over him and not even consider him as a buy. And while I won't even consider an unregistered horse, I will pay more for one whose papers are in order and has a traceable history.