It would mean being allied with all 12 clans by default, and that anyone moving against us would be personally insulting the Commander, so if anyone should take a swipe at us, we'll have Grounder support. Trade will probably be easier; if they're our allies, it'll benefit the clans to trade with us successfully.
The downside is that I'm not sure that everyone wants the alliance as much as Lexa does. If we don't cause trouble, they'll probably be won over in time, but most other Grounders know the Skaikru as the people who fell in Trikru territory and immediately (accidentally) declared war. Our history with the Woods Clan is bloody, mostly out of misunderstanding. We killed their biggest enemies (the Mountain Men), but they're superstitious, and they find Mt. Weather pretty creepy in general. Trust might take a while to earn, and if Lexa should die before we earn it, the next Commander might reject us or take a swipe at us. They don't see us as Grounders, not really.