So you're saying a single Diplomacy check can get someone to treat you as a trusted friend?
Well, it could in past editions. But no, that's not what I'm saying.
If, in the game, the party needs a Skill Challenge to join a merchant's caravan, there has to be a reason
why a single Diplomacy check doesn't do the job. After all, if no other issues are involved, and you can argue your case convincingly, a well-argued and convincing speech should be all you need.
So, typically, other factors will be in play. The merchant is paranoid and fears saboteurs, for example. So even if one person can convince him to let them join, you still need to convince him of the trustworthiness of the others.
Instant Friends doesn't change that. You rarely are just trying to befriend someone in a Skill Challenge. You are usually asking something of them - typically something that will cause some risk to them or their property. That's generally why one check is rarely enough, and more persuasion is necessary. The same remains true even once you have used Instant Friends.
If you believe that Instant Friends delivers the effects of a single successful diplomacy check, then it's going to work just fine when your players use it, fitting seamlessly into any skill challenge that you'd accept a use of Diplomacy in. Many other DMs may interpret it differently, leading to it bypassing skill challenges or being more or less useless.
I don't believe Instant Friends always equals a single free diplomacy check - you portray that as my interpretation several times, and I just want to make clear that this is absolutely not what I am saying.
It is the effect I would likely give it in many Skill Challenges. But that's already often how I handle powers in Skill Challenges - a fitting or clever use of a challenge will often earn one or two successes as appropriate. That doesn't mean I'm saying the rest of the power gets removed. A clever illusion might give a free success in a skill challenge - yet I'd hate if they changed an Illusion power to read, "This power grants an automatic successfull Bluff check!"
Which seems, basically, to be what you are advocating.
Don't forget that the subject may react negatively if the power fails or after it wears off. These are all things that were tried - some from the very beginning - to keep the old spells Instant Friends emulates from being problematic. They never worked consistently. Some DMs might really lean on the limitations, or have different feelings about what 'trusted friend' constitutes, and make the spell useless, other's might let it be far too powerful. It's just a badly formed non-mechanic mechanic.
If a DM ignores the limitations, that is just as much a problem as if they ignore the limitations on other powers. It is true that how important 'being a friend' will be can change from one DM to the next, but the same is true for skills themselves.
In Jon's campaign, my thief walks up to a barkeep and begins a friendly conversation in an attempt to get a discount on my meal. I go to roll a Diplomacy check, but Jon decides my charming conversation does the job on its own, and gives me a discount.
In Jim's campaign, my thief walks up to a barkeep and does the same. I go ahead and roll my Diplomacy check, Jim gives me a bonus for my chatter, and I roll well, and get a discount.
In Joe's campaign, my thief walks up to a barkeep, and I know Joe just cares about the roll, so I just declare I'm trying to get a cheap meal and roll my Diplomacy check. I fail, so no discount.
In Jack's campaign, I try the same thing. Jack says the barkeep isn't won over by my talk, and that you can never use Diplomacy checks to get someone to reduce any prices for you.
In Josh's campaign, I try the same thing, and he decides to run it as a full-fledged skill challenge require multiple Bluff and Diplomacy checks, and some knowledge checks about the bar's history, in order to get the discount.
Does the game fall apart because all the DMs run social interactions differently?
Of course not. Instant Friends doesn't change that, and doesn't need to be fixed any more than the core skill system needs to be completely revamped to ensure that all DMs run campaigns absolutely identically. It isn't possible to do so, and honestly, isn't needed.
Movement-based challenges are typically 'low level.' Take 'climbing a cliff' as a Skill Challenge, cliffs are cliffs, climbing is climbing, before too long, climb checks aren't going to be much of an issue even for those not trained in Athletics - bypassing such a minor inconvenience at 10th level isn't such a big deal. Social Challenges increase in difficulty more smoothly, as the beings you deal with can simply be much highter level. A utility of any level that /helps/ with social skill challenges is fine. One that bypasses them isn't really apropriate at any level.
Well, at least for myself, I have enjoyed running and playing in movement-based challenges at Paragon and Epic levels. At epic levels, you aren't scaling an ordinary cliff, you are scaling the flaming spiked pits of the nine Hells. Climb checks scale as appropriate - that's part of the game! Unless you can fly, of course, and simply bypass it.
It seems hypocritical to say that it is ok for certain powers to
clearly bypass such challenges beyond low-levels, but that a power that has only the
potential to bypass challenges (and, as I've shown, rarely does that) is unacceptable.
Really? So 'make an Arcana Check instead of a Bluff Check,' say, would vary radically from one DM to the next? Skills aren't the most tightly-defined part of the game, with DMs often needing to make judgement call about which skills might be used to accomplish what tasks.
That's what I mean. A power that gives me +5 on a Diplomacy check might be great in one game - or useless if the DM doesn't let Diplomacy accomplish anything. I like having a power that I know will accomplish what I want - Instant Friends seems
more defined for me than most skill-based checks, since it lays out clearly what it accomplishes, along with what is needed for it to succeed, and what limitations the power has.
It will still be subject to DM fiat, but honestly, not as much as the use of skills themselves.