A lot of people are, if not opposed, at least strongly skeptical of imports. Last week, there were a few opinions on the subject, including one that offered, "Never trust robots," as a policy statement.
Naturally, this was the same week I found Baltimore City's Open Data Catalog, which is full of public domain data, some of which could be very useful to OpenStreetMap. I decided I wanted to try importing the landuse data, since that would liven up the city's OSM data and wouldn't, I thought, need too much work to integrate it into existing data. I figured I'd convert the shapefile to an OSM file, tag every landuse with some "unprocessed" tag, then go through the whole city and make sure any existing landuses were incorporated into the process, so no one's data would be lost or ignored. I planned on testing out this process in a few areas of the city to make sure it was feasable, emailing the talk-us list and the other people who had contributed to Baltimore mapping, and then proceeding if there weren't any objections.
I didn't get that far. The shapefile was such a mess relative to the topological quality of data I would expect from OpenStreetMap that I decided it wasn't worth the effort it would take to clean it up. There were tons of places with pointlessly overlapping landuses, others with overlaps that might or might not have been pointless, nodes that ought to be shared in OSM but which weren't quite close enough in the shapefile to have been merged during the preprocessing, and quite a lot of tiny slivers of areas an inch or less wide.
This more or less matches my experience with the National Hydrography Dataset. It's decent data for a lot of uses, but on its own, it doesn't match the quality of what can be put into OpenStreetMap. In the NHD, streams can be misaligned by ten meters or more, and an straight import wouldn't address the manner in which waterways interact with roads--either going under bridges or through pipes. When I'm mapping waterways, I use the NHD as an overlay for my state's aerial imagery to give me a rough idea about the directions and names of waterways, but I trace their alignment from the imagery, not the NHD.
Similarly, I'm working on making a rendered tileset that Baltimore mappers can use to see the city's landuse and then make their own judgements about how to bring that into OpenStreetMap. (In my use of that rendering in conjunction with aerial imagery, I can also tell that the city's data isn't always exact about property lines, although it's good enough to use as a pointer, at least. Just like the NHD.)
In summary, my experience has been that OpenStreetMap demands precision in ways that traditional GIS doesn't, particularly in the topology of the data, which is why raw GIS data is often a very poor fit for OSM without significant manual work to adapt it. That's a very large argument against most imports.
Of course, I'm not entirely dissuaded. The city also has a shapefile with block-by-block addressing. I still have visions of importing that data into OSM, because I think it would be tremendously useful. I don't expect it to be easy, though, and it might not be feasible at all.