They're more floating/filling up the mid-water as they expand and grow, and don't naturally root so often. At least, the don't root in my pond. They suck up C02 by being closer to the surface, so can grow and get thicker and denser, much more quickly than if they grew from roots in the substrate, so they're quite a buoyant plant naturally anyway. At least in my pond, it'll grow so dense midwater once it gets all that spring and summer sun, the anacharis and hornwort all intertwined, that I have to remove loads of it periodically throughout the season to give the fish some swimming space!
We can plant stems so they grow upright, but I've always found that it doesn't grow as well or quickly when planted to grow upright, and the roots are thin and don't go deep, so they're easily uprooted. Especially with substrate disturbers! I have cories, who also uproot plants that aren't well rooted.They'll always uproot it with zero effort at all, so you have to weigh it down somehow to keep it down.
I find the plant weights the most effective too, but as you say, eventually the bits that are clamped down are going to rot, then the rest of the plant floats to the surface. Or the plec digs around it or knocks against it, it'll break at the weak points and float.
I only tend to use it floating now, mainly for water quality purposes. But if you want it growing upright, use weights or ceramics, and be prepared to have to re-weigh them down again, so depends if you feel like having to do that whenever the lead weights need to be re-wrapped, or the plec digs them up.
