Not common?
It's interesting that you think it's not common. While I was learning astrology I came up with the exact opposite reactions. I thought house rulerships were as essential bread-and-butter astrology as sign rulerships were.
One of the first things I remember reading about (12 years ago now) was that the Ruler of the Ascendant was a major power player, or "Lord of the Chart" and acted as an ambassador for the Ascendant. Subsequent study revealed that just as the Ascendant is the first house cusp, each other house has a cusp, and the ruler of the sign on the cusp can act as an envoy of that house. Everybody who has an empty house and issues pertaining to that house wants to know where to look for answers. The house ruler can help.
Also -- and this is why I'm confused as to why house rulerships aren't obvious -- there are 12 signs and 12 houses -- each sign has an affinity for a house. Signs have rulers -- so houses have "natural" rulers too. Mercury and the third house just go together. Saturn and the 10th, as well.
These house rulerships are also used *extensively* in horary astrology -- both the cusp rulers and the natural rulers of the house that the question is about play a part in determining the outcome of the answer.
Furthermore, sign and house rulerships can be used to calculate a sort of "hierarchy" of planets -- which planet has the "loudest" voice, or freeest expression, as it were. I'm sure you're aware of mutual receptions -- planets in each others' signs -- well, this can apply to houses too -- where planets, which may not be in mutual reception by sign still rules each others' houses. Some astrology programs, like Astrolog, have this feature built in.
A couple examples: Jupiter in Cancer in the 7th and Mercury in Capricorn in the 1st, in a chart with Sagittarius rising. Even though neither planet rules each other by sign, Jupiter rules the first house by virtue of its Sag cusp, and Mercury rules the 7th by virtue of its Gemini cusp. So there's a form of mutual reception there. If there's an opposition between the planets, it outta soften it up a bit.
Alternately, The Sun in the 10th and Saturn in the 5th, in whatever signs. Each other has a natural affinity for each other's house -- so there is a distinct connection.
In my opinion, house rulerships don't *seem* to be as strong as sign rulerships, but I'm not 100% convinced of that.
Anyway, I just thought I'd share for the class.
-V-