Are there disadvantages to the house rule that you always end before the starting player?
The game with all the expansions has 3 ending conditions, all of which require you to keep an eye on the number of cards remaining.

Those endings are:


All Provinces have been bought.
Any 3 (2-4 players) or 4 (5-6 players) piles have been bought.
(New in Prosperity) All Colonies have been bought. 


Short version:
It would discourage players from buying the last card to end the game if it's not a Colony or Province.

Long version:
The idea is that you watch the piles as they go down and adjust your strategy to compensate.  For example, if I think my score is good enough to win, it's to my advantage to buy out the last pile needed to end the game, even if it's something I don't want.

Requiring the game to keep going around the table after that would likely discourage me from buying said card; I would rather buy more higher-point cards, hoping that someone else will end the game.