What is the typical lifetime of a Phal root?

[cythaenopsis]

I've seen many examples of Phals with roots in various stages of life. Some are new with a white shroud, some bright green, beige, brown, black, and shriveled. Does a root typically last for one bloom cycle? I'd heard it is a good idea to repot a phal about once every other bloom cycle, so that you can cut off the dead roots and freshen the potting mix. Will some roots die off at any time during the seasonal cycles, as new ones emerge? How important is it to prune off the old ones? I guess in the wild, there's not as much coverage over the roots so they'll fall off naturally, and that in a packed pot the usual method of shedding old roots is hampered... and that's why we need to clip them?