While I have not done a close study of Montenegro's military capabilities and likely defense spending, I am going to guess and say that it does bring much to the table. Montenegro seems to have all of one infantry battalion, two patrol boats, maybe some planes, and not much else. It is already well under the 2% of GDP for defense goal at 1.3%. Montenegro does not occupy a key position in the world (however defined) except perhaps as a handy location for smuggling stuff into Europe (untaxed cigarettes is or was a big income earner).
But it does get NATO closer to having a map of Europe that is whole and free and happy and blue--that the only parts of Eastern Europe that are not in NATO are Montenegro, Bosnia, Macedonia and Serbia. Only the last is not in some real or mythical line to get into NATO. Having a map without any holes (except for Switzerland and some micro-states) has been a key objective of some folks.
What does Montenegro take away? Given its location, it is unlikely that NATO would have to defend it from an attack, so it does not stretch the credibility of Article V. That is one of the key reasons why membership for Ukraine or Georgia is a bad idea. Adding a weak, probably corrupt Eastern European country to NATO does not change things much--NATO already has Albania, Bulgaria, Croatia and Romania, so Montenegro does not change things much on that score.
So, is Montenegro's membership status entirely irrelevant? Not at this moment. Why? Because it gives NATO a chance to stick it to Russia*, who has opposed the inclusion of Montenegro in the alliance. This shows that NATO membership is not subject to Russian vetoes. And that is a good thing.
So, for the moment, Montenegro is slightly relevant. And if it tried to block decision making at the North Atlantic Council, it could make some noise. But Montenegro does not add much to the alliance nor does it really stretch the alliance.
* This article says that NATO does not allow in members that have border disputes, but that is mostly not so true. The key for Georgia and Ukraine is that their border disputes are with Russia, and that their inclusion in NATO would stretch the credibility of the NATO commitment to the breaking point.