The only "stupid" question is the one you already know the answer to. Generally, a "sucker" refers to shoots of the root stock which begin growing from beneath the bud union, where the stock and the grafted variety you meant to buy, join. If you remove and propagate that stock sucker, yes, you do have an "own root" rose but it is the root stock and not the named variety you purchased. If you root one of the stems or canes of the named variety which is budded to the stock, then it becomes an own root plant of the named variety. "As good as any other own root?" Mostly, yes. Probably as good as any other own root plant of that named variety. Presuming that variety performs well in your climate, soil, water and culture, it should succeed. If the variety isn't one which performs well own root or doesn't perform well own root in YOUR climate and conditions then you may not have as good of a plant own root as you do in the budded plant. Should you do it? If the variety is patented and still protected by that patent, then probably not. It is not ethical to propagate a rose which is under protection of a plant patent. It's theft of intellectual property. If the variety isn't patented or otherwise protected, sure! Why not? You won't learn without experimenting.

Thre has been a lot of discussion about propagating patented plants for your own use, and the legality...and enforcibility of such a law. I would find it hard to toss extra fans of a patented daylily that needed dividing.
There is a nursery that sells own root roses, and I suppose they only do that with stock they are sure is sturdy in many climates, I have had good luck with their products.
I remember once many years ago, someone gave my mother a bouquet of beautiful blush pink roses. My father took one, cut off the bloom, stuck it in the ground behind a bush where it would get no sun and covered it with a 2 gallon glass pickle jar. It rooted and grew, turned out to be a prolific climber.