This was just an example for a everyday situation where a Charisma check and thus "Friends" could be useful. The situation was chosen to show the problem with the duration of "Friends" when negotiating.
Why would "Friends" give an automatic discount on everything? It only gives advantage on Charisma checks. As there are almost no rules on skills it is up to the DM to decide how a successful or good Charisma check would benefit you while bartering.
If Friends can be used painlessly to improve a character's bartering ability, it is an automatic discount on a routine action. Characters purchase items almost every time they are in town. We generally don't role-play these at all, as it's a rather boring role-play, unless they're trying to buy something outlandish. However, if a player knew that he could use the Friends cantrip to get a 15-25% discount from shopkeepers (say a 10 minute duration before the spell wore off), these things would go like this:
DM: It's morning, what do you do?
Friends Caster: I go to the store to buy healing potions.
DM: Ok. Those are 50 gp each.
Friends Caster: Great. I cast Friends and try to barter with him. I roll a 17 on my persuasion check. What do I get?
DM: Umm, I guess you pay 80 gp. But he will be pissed at you in 9 minutes.
Friends Caster: Awesome! We're on our way to the dungeon anyway! I'll just buy from a different shop-keeper next time.
DM: It's fun doing that every time you meet a new shop-keeper.
Obviously you see "Friends" more as a getaway spell. I can understand that, as it is probably the only useful situation for the spell. But I think it's a pity that you can't use the spell as a "diplomat" while negotiating.
Well, also as a get-in spell. Maybe you are sneaking into an enemy city or fortress. The rest of your party is hiding in the back of the wagon, fully armed and ready for action. You've disguised yourself in an enemy uniform, and all you need to do is convince the sentry at the walls that they don't need to check your wagon too carefully. Once you get through the checkpoint, you can hope to disappear while the guard raises the alarm and locks the whole place down. Now, maybe you say, "I dunno. That could take a while." Which is where the check comes in. If you kick butt on your check, the guard waves you through without questions. You have about 45 seconds, or about 8 combat rounds, to disappear before the guard realizes that he's been had. If you squeak by on the check, there's some back and forth with the guard. You get through ok, but while you are still in view of the gate, the guard realizes that he's been had, and sounds the alarm. Now you can either try to run away, or stand your ground. At least you are on the other side of the walls, where you stand a better chance than out in front of the gates.
That's a risky use of the spell, but it's fun, and it might be your only option if nobody else in your party is able to pull off disguises. (Maybe you are the only one with the performance skill?) Maybe you are prepared with a charm person spell in case things get out of hand, but you'd rather save your spell slot if you can. Maybe it's not a super well fortified position, but you'd still rather fight from the inside than the outside.
I see this spell as having creative applications. I don't see the fact that it doesn't have a simple application in a mundane situation (bartering) as a drawback. I don't mind that it doesn't help with diplomatic situations. The name might be a bit of a mis-nomer, as it is more "False Friends." It's a con spell, and, as a cantrip, it's fairly limited in scope.
I think the part with the murdering was just a joke.
Sure, but the attitude is that the hostility is a game-ender. My point is just that having somebody be hostile towards you doesn't have to be a show-stopper in plenty of situations. Hostility can be pretty minor, or pretty catastrophic.
It is not about whether the conversation lasts 59 seconds or 61 seconds. The point is, in just a minute you can only have a very short conversation. Any conversation that would involve some sort of persuasion would last longer otherwise it wouldn't be much of a persuasion. Therefore "Friends" can't be used for negotiations or deception where you have to explain things.
It's not about whether the conversation lasts 30 seconds or 2 minutes. That's my point. D&D is not the place to precisely time out the length of your interactions. But, yes, it is only going to work on fairly simple interactions. Distractions. Getting somebody to accept something at face value. Intimidations. "Let me in. Let me through. I am a member of your army, just as my costume would suggest. There is nothing strange about me leaving this satchel here. Talking to me is more interesting than standing at your post. I think they went that way. A boy fell down the well and if you don't climb down there this second, he will die!" If you need to explain a lengthy concept, or have a complicated series of back and forth interactions, Friends probably won't do it for you.
The spell is meant to give you enough time to convince somebody of something simple, and then, if you do everything right, put a couple of hundred feet between you and the person you've just pissed off. (Or a couple of crossbow bolts in the back.)
What would be wrong with that? With a simple Charisma Check I can con a guard and run 100 miles.
Sure, but you have a much better chance of succeeding with Friends.
EDITED TO ADD:
I am totally open to the idea that Friends might need a tiny bit of buffing. Personally, in my game, I would rule that Friends opens the door to kinds of persuasion that might not otherwise be available. So you might be able to convince someone of something with Friends and a Charisma check that you might not be able to convince them of with a simple Persuasion check. Personally, I think that opens the spell up for use in situations where the risks (hostility before you can get away) keep things interesting. My problem is with measuring the effectiveness of Friends by how well it handles negotiation with a merchant. I don't think that's what the spell is really for. The spell is for quick-con interaction encounters, and its effectiveness should be measured in those types of circumstances.
