Please, please, please don't suggest that more people take advantage of this loophole! The Vancouver/Portland commute has already rendered I-5 useless, and the residents on the Washington side are currently lobbying aggressively for a new 12-lane bridge (whose cost would be paid largely by Oregon) across the Columbia to ease the congestion without taking any steps (light rail, tolls, etc.) to reduce the amount of traffic.
Portland is what it is in part because residents of the city and Multnomah county are willing to pay relatively higher taxes than those in neighboring areas for a higher level of service from the city. Our transit + public services are pretty good, but the folks who drive down from Vancouver and ride the MAX through Fareless Square to shop without paying sales tax or income tax are just freeloading.