maelstrom is farmable, but keep in mind that on WR (if we're even still talking about her) its probably not the best item rush, even on a safelane WR. you'd get more farm out of an oblivion staff and it just doesnt match the way you should be playing.
i wouldnt do a maelstrom rush on any hero really though. 5-6 minutes is ambitious, you're hardly ever THAT uncontested, 8 minutes, maybe, if you skip finishing your phase off, which gives you weaker laning.
i think the main point is, that its good for what it costs. and you're never done farming. if you've got nothing better to buy, maelstrom gives you the potential to farm while fighting. even if you're not actively farming, the passive pushing is pretty usefull.