Well, speaking from a game in which I'm one of the stronger players and was surrounded by several weaker players (weak in terms that they had much less territory and could not do much except probably break someone's [like mine] bonus) I was more than happy to form alliances with them to let them keep their bonuses near me and also have them help me (and themselves) by fighting my (stronger) opponents.
But you are right, in that weaker role you have to be cautious. This was an 8 player game where 4 players had become "top dogs" and 4 were on the verge of elimination. I was allied with 3 of the "small dogs" but eventually at some point 2 of them decided to test their luck and come after me - and then I quickly eliminated them because I wasn't going to deal with that. One of the players was so weak that yeah, he probably wasn't going to last much longer anyways, but the other - he had a decent bonus and was starting to make a comeback, he just got greedy. I think if he had kept our truce, he could have really taken some ground against our common opponent and gotten himself back into the game because I had no interest in fighting him at all, I was much too busy with other opponents so I was perfectly content to share a border that didn't need to be defended.
The last ally, I lost because he was losing to the bigger guy he was fighting and was on the verge of being eliminated, so I took him out to get his cards (hence the other thread I started lol).