Eh I have actually accidentally done that to people before. I used to do all my events in the Norn starting area because I knew them and knew the timing of them really well. It wasn't until I two shot the Broodmother and looked up to see a bunch of low levels had been running over that I realized I was kind of screwing things up for them. I wouldn't doubt there are some people out there who do it maliciously though.
As for the Mentoring system in Wildstar, it seems interesting. I think some people might do it because they like doing those sorts of things. However, I think most often it would be used by higher level people who have friends starting later than them.