A custom that (where I live) is becoming more honored in the breach than the observance. It makes driving very much more dangerous.
In Britain they have a sardonic nickname for people who do this: CLARAs. "Centre Lane Residency Association".
To your point, I think the middle lane of a three-lane road is, ideally, the correct travel lane. Cruising there at the prevailing speed leaves one lane more lightly traveled for entering and exiting, and the other for passing. Predominantly using that lane minimizes lane changes, which are the most dangerous driving moments. You're right, though, that the strategy breaks down as traffic gets heavier, and gets ruined entirely when (as under discussion in this thread) people gum up the supposed "fast lane".