Yes, if the amount is over the state's filing threshold.

Sometimes

And they are likely to get back some of the MD nonresident withholding when they file the MD return.

Withholding from what?  There were no MD taxes withheld on the W2 and there aren't any withheld on the K-1.

Sometimes as in if there's enough income to require a tax return?  Or sometimes for another reason?

There should have been for the K-1 income.

Even if no distributions were made?

States tax on income earned, not distributions.

MD doesn't care. If there is income on the K-1 they want their cut and they do it with "withholding".

So you're saying that MD requires you to withhold taxes on K-1 income?  I haven't seen that.  I read that the business pays a "pass through entity nonresident tax" if they have nonresident members.  I guess the concept is the same as what you're saying, but I wouldn't consider that withholding.  

Yes that's it. Excuse my "slang"

If the entity filed a composite/group nonresident return your client does not have to file a MD return.