(Try not to say the obvious answer of having friends is what keeps Buffy a good Slayer and alive, try thinking about the negative side of it as well)!
Just because something is trite doesn't mean it's not true. You know I can't think of an example in something like a graveyard fight, Buffy being unable to vanquish a foe because her friends were there. Normally three things happen in those situations: (1) the friend (frequently Xander) gets comedically knocked out and Buffy continues to fight until the fight is over and then picks said friend off the ground; (2) the friend saves someone or stakes some vamp or (3) Buffy, in saving the friend, vanquishes the foe. However, it's pretty much never a minus in a situation like that. I think Riley distracted her once or twice when he was being all Super Soldier attention-grabby but that's it.
Most of the time, Buffy's friends are in the biggest danger because they were researching for Buffy or just living their lives. Typically, it's not being in a fight that really hurts Buffy's friend. It's Tara sitting on a park bench in
Tough Love or precisely, Buffy leaving in a snit because she doesn't want to protect the civilians in
WSWB and them being captured anyway or Willow being kidnapped by Mollock because she embraced him as a pen-pal. In that case, Buffy taking time out of her day to fight for them is exactly what she should be doing as a slayer. Protecting Willow from Mollock or Xander from Miss French is the equivalent of protecting Cordelia from Marcie or Blaine from Miss French- the lifeblood work of a Slayer. Protecting Buffy's friends in the line of studying and researching for Buffy is value-added for Buffy because she's getting magic or research help that she couldn't get on her own.
Most of the time, the effort that Buffy expends for her friends is less than negligible compared to the help they provide. So Buffy dives to protect Willow from bullets in
Primeval and doesn't rappel down the elevator shaft as quickly as she could alone? Completely negligible compared to Willow providing indispensable witchy power to take down Adam and indispensable help to find Adam on the computers in the first place, not to mention the emotional support and rebonding that gave Buffy much more power and commitment on the elevator shaft. Buffy rushed to save Xander from the Ampata the Mummy and then she ended up needing Xander to defeat Ampata with her attempted kiss. Cordelia was annoying and burdensome to have around in the SlayerFest of
Homecoming but bottom line, Lyle Gorch could have taken unconscious Buffy if Cordelia was there to intimidate him into running away. That's the much more consistent pattern.
