I didn't play with alliances until recently. Gave my first try at Apollo, playing as Brazilia. Thing was, until now I really hadn't been attacked by other players (I always play Single Player), and was more concerned with aliens and the odd uppity settler so my military force was minimal. And BOOM! Sneak attack by Polystralia and ARC going for my throat as I was down (This was also the first time I'd seen the "carpet of Doom" in BE).
So, the next games, I went back to Soyuz and made alliances when possible. And my third game in, Hutama was squeezed right up next to me and teching up so Alliance, Alliance, Alliance! It kinda worked, as PAV and AU, my allies, were wedged up to Poly again and strong in tech and affinity. Which, BTW, I wasn't. Bad terrain, poor planning and aliens galore... I was lagging, barely level 8 in Supremacy. But I was never attacked by Hutama, which was good as he was at least 13 in Purity.
And then Hutama completed the warp gate, and PAC and AU declared war on him. And suddenly I was at war with Hutama at 13 Purity. It... It didn't end well. So, alliances can be useful in my experience, just by intimidation alone. But they can be a double edged sword.