Before there's any serious benefit there's got to be some nastier injuries that can come out of everyday rough and tumble, like breakages if it's an impact damage, fevers if you're in a swamp, maybe infections if you go long periods with damage and no first aid.
Once you've accumulated these nasty long term conditions, they should debuff stats, and possibly move speed / stamina / hunger (ooh, I like that one, what if all you could eat is bread / apples otherwise you throw up and lose FEP's)
These long term conditions should typically have a chance of clearing themselves after a certain amount of time (the body is an amazing self repairing machine) but chances and recovery times should be substantially reduced by visiting a proper healer, who can apply splints, know the correct way to dig out necrotic flesh and apply leeches/poltuce's/flame to sort out infections.
Possibly also allow these healers to speed the recovery of HHP damage if the character being healed spends large amounts of time bedridden being tended to at regular intervals. (possibly allow them to log out while receiving treatment but still have their character in game.
Having such things in play also suggests poisons
