Borrowed Heros Leader Ability

yes if the character borrowed has a leader ability and your ally has unlocked it you will benefit from it (pending eligibily of your squad and chars) that's why you love having an ally with palpatine or dark vador when you have a palpatine squad !

There are certain characters -- like all the Bounty Hunters -- that say, "When this character is in the Leader slot and not the ally slot..." in their leadership and/or their unique. 

For the Bounty Hunters it's because you don't want 2 Contracts at the same time. For other characters there are other balance reasons. This shouldn't affect you too often, but you should be aware that these carve outs exist. 

Also, remember that squads are hard-limited to 6. So if you have an ally, you can't have a summon. This is true even AFTER one of your own character's die. If the ally dies, that frees up the slot so you can summon someone, but if one of your own toons dies, summon still won't work. 

TLDR: You get the basic leadership, but having an ally can sometimes come with a cost. Even so, You generally want an ally every time the option is available.