depends, personally i have three class's class item, want to switch allegiances until i get the other possible six, ie 3 for each char, unfortunately without old nightfall/bounties going to faction only it'd take forever, only have about 700 motes and 350 coins so wouldn't even reach rank10 on One new allegiance that route, it was ok getting them shortly after TTK landed, novelty's worn off alas, so short answer to your question, not really worth it 'atm', long answer, yeah, who knows how it'll change in some future dlc/update so having high/at least lvl25's in factions could be a good place to be, fingers crossed.

All possible is anything from the faction. So helmet, chest, shoes, gloves, class item, and whatever weapons the person sells, all of these with random rolls (if the person doesn't sell a scout rifle, the faction doesn't have one), as well as the 3 faction ships and 3 (4?) shaders

Ranking up gives u a chance at a new roll on a gun

All emblems,shaders,ships and exotic cloak at rank 25 with dead orbit after that there's nothing else you can get but if you decide to pledge with a new faction you can no longer wear your exotic cloak,bond or mark

If you haven't gotten a ship you wanted. And it's all RNG except for faction quest class items. I'm rank 45 with Dead Orbit and have every shader but missing 2 ships.