With the lathe running, the only disadvantage I can see, is the lack of tool rest. I haven't done it with a hand held card scraper, but I have done it with a hand held standard lathe scraper. Being hand held, the tool will tend to oscillate between long grain and end grain. The result of this will be an out-of-round final surface. This may not be a big disadvantage for some turning, but where geometric integrity is a main component of the turner's style, it works against a successful end result.
-----odie-----