Being a faction leader myself, I don't spread myself wide to other factions since it means I'll end up with conflicting interests, needing to be in two places at the same time, etc. Other players can manage this better, and all the power to them. Being in other factions does allow for friendlier interactions between forces as you can have the case where a player knows what it is like to be on both sides, trick though is ensuring that the actions with one faction remain independent from the actions of another.
My thoughts are that a player can have chars in any faction as long as they can properly separate the roles. However, leaders should only be the leader for one faction, leading two is a conflict of interest. But if a leader of one faction wants to have a char in another faction and can properly distinguish the roleplay of the two then sure.