It would be incredibly difficult to make Columbus Circle express. You'd have to effectively shut down service on the for months at a time, and completely reroute tracks, or make trains bypass 66th and 50th while on the local track, which is pointless, confusing, and would lead to incredible traffic and delays. Also, you should definitely add 125th St to that list, it's a major station and the line shouldn't go 60 blocks without stopping.
The current system works very well, since almost everyone commuting is commuting into Manhattan (or vice versa in the PM), and trains come frequently enough that it isn't a pain.