Committing vs publishing
Fundamentally, a commit creates a revision, and a revision is something we want as part of the permanent record of a file. However, a lot of those revisions are not really meant for public consumption. When I’m working on something complex, there are a lot of points where I want to freeze frame without actually telling the world about my work. Subversion understands this perfectly well, and the mechanism for doing so is branches. The caveat is that this always requires server round-trips, which is okay as long as you’re in a high availability environment with a fast server. This is fine as long as you’re in the office, but it fails the moment you’re traveling or your connection to the server fails for whatever reason. Subversion cannot queue up revisions locally. It has exactly two reference points: the revision you started with and the working copy.
In general though, we are working on high availability environments and making a round trip to the server is not a big deal. Private branches are supposed to be the solution to this problem of work-in-progress revisions. Do everything you need, with as many revisions as you want, and then merge to trunk. Simple as that! If only merges actually worked.