The house player rule has more to do with waiving membership requirements than with ratings-appropriate pairings. (Unlike match checking, it doesn’t look at ratings, as it assumes the TD has already done that, as per the next paragraph.)
However, the recommendations in the rulebook for house players is that they be ratings-appropriate for the score group they’re in. (It is worth noting that what is ratings-appropriate can change from round to round, another challenge in trying to check it during event validation. You could probably make several interesting quiz questions on this issue, if quizzes in the Forums become a reality.)
The house player is there for the convenience of (the players in) the tournament, so ask yourself this: If you were a 1200 player who was the odd player in a round, do you really want a rated game against, say, a 1900 player? Some players will relish the challenge, other might not.
The main reason we currently limit it to one house player per round in a section is because there’s seldom a good reason for having more than one house player (to even out the number of players in that round), and this is to prevent its abuse.