I've played through the Telvanni more times than any other guild in Morrowind.
Even the most isolationist person would surely recognize a danger that posed a threat to themselves! Even a magelord sitting in his study for hours on end, I'm sure if a servant came along and said "Oh, master there's thirty Argonians outside with their spears ready to prod us!" The mage lord give enough crap to walk outside and turn them to ash because the danger is to themselves.
I'm not saying the Telvanni would be like "ZOMG SAVE TEH WURLD" but any of them would be like "Oh better stop these lizards before they come for me."
Like in Oblivion how one of the rumors talked of the Telvanni closing Oblivion gates. The same exact principle. If the Telvanni didn't care enough, why did they actively close Oblivion gates?
See, you say the part about the Nerevarine and Morag Tong, when in Morrowind, the only Morag Tong agent that even gets a contract for a Mage-Lord is the Nerevarine, and quite frankly, you can't say that they are not powerful based on the fact the Nerevarine could kill them, as the Nerevarine slapped around 3 false gods.
Basically, what I'm saying is yeah the Telvanni wouldn't be world saving superheroes but they would protect their own territory (like mentioned in Oblivion.) And I'm sure an average Telvanni mage is a good match for about 5 Argonian foot soldiers provided they weren't in spear distance.