Curious, why are legumes bad for you?
But a potato is a legume and not a veggie just like corn is a grain and not a veggie, therefore shouldn't fall into that same "Hey eat LOTS of these!!" category. (Sweet potato, by the way, is a root veggie, like carrots and turnips.)
You could, however, google "lectin" or "potatos" and "leaky gut" to see why some people have issues with potatoes, and may not even realize it's the potato who is the culprit.
Here's a nice little write-up on legumes as a whole (including beans) and why it may be wise to limit them:
http://www.marksdailyapple.com/beans-legumes-carbs/
and some info from the same writer on potatoes in particular:
http://www.marksdailyapple.com/paleo-potatoes/
http://www.marksdailyapple.com/potatoes-healthy
Note where he talks about how eating potatoes seems linked to joint pain (for himself) - and yet he still eats them.
I don't think potatoes are necessarily horrible and need to be avoided at all costs, but I don't think it's the sort of thing people should be shoveling into their mouths with the same frequency as, say, spinach or broccoli or, really, any given other veggie. Fine in moderation like most things.
But just plain not a veggie.