Importing other people's data into your own is almost always a very bad idea. Too often, people are merging other people's GEDCOMs into their own, trusting that other people did theirs correctly and proliferating what are called "junk genealogies".
Not only that, you WANT to keep your work separate. If relatives send something to you, why go through days or weeks of work to merge theirs into yours, only to have them send you an updated GEDCOM six months later. That time could be much better expended doing your own research.
With regards to online trees, I believe you should only upload your own research, and not other people's. If they want their information uploaded, they should do it, not you. If they don't have the technical capability and ask you to do it for them, then you become the "trustee" to their data, and you should not pick and choose which parts to put up. They should send you what they want put up. Then the issue of how to import sections of their tree becomes moot.