I'm not exactly an expert on the subject, but I remember hemp having an awful lot of uses in Puzzle Pirates... Anyway, a
trip to everyone's favorite non-DF wiki says...
The only thing I'd otherwise think to add is that hemp oil was used as a paint base. It can also be used as a cooking oil and has a high omega 6 and 3 fatty acid content. It should be useful for anything other vegetable oils are used for.
Other notable things are that hemp is basically a weed, and as such grows well in dry areas, grows extremely fast, and is pest-resistant.
So I'd say that if we ever get paint, a hemp-oil base for that paint is reasonable. It is also a reasonable cloth type or for paper making, so it would otherwise be basically the same as a rope reed.
The types of hemp used for fibers and the types of hemp that are marijuana are two different types of plant, however, (they seem to either have been bred for their fibers or for getting people high, so it's a one-or-the-other deal), so the "get high" hemp, if we ever have those high dwarves thing come around would more realistically be a different type of hemp.
All-in-all, there isn't much now to recommend adding hemp over just using pig tails and rope-reeds given what the game can do with them, however, so I think it would be better to wait until Improved Farming comes out, and then add it in as an alternative type of crop.