There are four main issues with plus-sizing that I can think of.
First of all is the increased cost of wheels and tires. A 17" tire is a lot more expensive than a 15" tire.
Second is rubbing. A larger wheel will tend to have a wider width, which, if the offset is not correct or the wheel is too wide, will rub on the fender liner, the inside of the fender itself, and in a worst-case scenerio, the strut itself. I have 15x6 wheels and they rub on very large bumps if there's more than two people in the car.
Third is the ride. To keep the same overall diameter with 17" wheels, you have to get tires with a thinner sidewall. This usually means a 40-series tire or even less in some cases. This thin sidewall will transmit much more of the vibrations and bumps in the road into the car.
Fourth is weight. A 17" wheels is going to use much more material than a 15" in general, and will usually weigh more. Even if you have two differently-sized wheels that weigh the same, the larger wheel will have more of a detrimental effect on the vehicle's performance than the smaller since there is more heavy material farther away from the wheel hub. This performance detriment effects not only straight-line acceleration, but braking, and handling as well.