If there were dozens of such planets it would represent a considerable extra supply of resources available in the solar system as a whole- there's only so much
solid rocky matter in the inner system even if you seriously consider taking apart Mercury, Venus, and so on for building materials. But they'd be very hard to access, and it would be a matter of centuries or millenia to get any mileage out of them.
There's a catch when we talk about the idea of rogue planets orbiting the Sun at distances on the order of tens of thousands of light years, though- close passes with other stars.
Apparently, there are a few planetary formation theories that have a lot small planets forming in the inner solar system. These planets, called Ogliarch, in the models, are tossed out of the solar system (meaning any Rogue planets we find within 1 - 2 LY of our sun might have started off inside our solar system), or tossed into an area roughly 1000 - 10000 AU out. 'Planet 9' could be the inner most of this group. There could be hundreds, or more, of these worlds floating in the outer edge of the solar system.
If bodies are orbiting the Sun at radii on the order of ten thousand AU out, the probability that they'd eventually be yanked away by another star is pretty high. The Sun has close encounters with other stars... not often but it's happened many times during the history of our solar system. I wouldn't expect orbits that far out to remain stable, although it's entirely possible that we would steal planetoids from them just like they would from us.