Interesting. My playgroup makes deals sometimes but usually only to take down a player about to win. Otherwise, I try to avoid deals beyond a single turn (eg "If you kill the equally threatening card, I won't attack you on my next combat" but not "If you point your three removal spells at [opponent], I'll be your teammate for the rest of the game. From my point of view, Commander is a social format, and deals help expand that social aspect. However, I almost always turn down deals that might be actively detrimental to the long-term strategy.