The only power restrictions that I know of are "if a power gives a benefit from attacking an enemy, then the enemy you attack must be a viable threat or the power doesn't trigger" and "powers that target creatures may target objects at the dm's discretion".
First, a disclaimer: I know (and assume everyone knows) that this section is about recommendations, not set-in-stone rules.
You're misquoting the recommendation in question, however, in a relevant (and oft-mistaken, hence this highlight) fashion.
When a power has an effect that occurs upon hitting a target - or reducing a target to 0 hit points - the power functions only when the target in question is a meaningful threat.
The purpose of the limitation you're referring to is clear; if succeeding vs. an opponent grants you power, the success must be meaningful. The emphasis on success is important, however: if some rule permits an attack and an incidental effect, or, vice versa, an effect and an incidental attack, and the effect and attack don't require
succeeding at some challenge, then the level of "meaningfulness" of that challenge isn't relevant.
For example, twin strike permits shooting two arrows. The second arrow may be shot regardless of whether the first hit, and you should be able to use this power's second attack regardless of the foe (or indeed, even if the "foe" is an apple on a jester's head). Twin strike may seem like an obvious case, but other powers get more complex, and it's easy to lose track of the real purpose of this guideline.
So, it's not that
powers don't work against meaningless threats (or outside of combat) - it's thatsuccesses against meaningless
challenges are themselves meaningless. If a power requires some success and you don't require the challenge to be meaningful, you might as well not require success in the first place - thus the presumption that effects
that trigger on a hit or dropping a creature are written with the assumption that the threat is meaningful.
And it's definitely
not a rule, it's just a guideline for interpreting the assumptions of rules. Were it a rule, you couldn't even deal damage to a rat: after all, the effect (namely, deal damage to the target) of the "melee basic attack" power occurs only on a hit, and the target in question isn't a meaningful threat!
The guideline is intended to avoid absurd scenarios like a warlock sacrificing a swarm of flies (each individually cursed, of course) by setting the jar on fire to gain a huge number of pact boons, or by a fighter dealing damage to the unhittable foe by cleaving through after hitting a rat. The rule is
not supposed to
create absurd situations where effects that have little to do with the target somehow stop working if no meaningful threat is around or where effects that
make sense even versus insignificant threats stop working.
The DM should exercise common sense and take into account the setting and world he's trying to create. You can't apply (or ignore) this guideline in an absolute sense without causing ridiculous scenarios. Something that makes sense in one world may not in another; that's OK.
To the OP: Suppose in your world the spirit companion has a will of its own - and only helps the shaman when (s)he's in need, you have a world where you should not allow him to summon the spirit companion outside of combat. Most DM's should just allow it, however: it's clearly not directly related to any target, so there's no intrinsically obvious explanation for why it doesn't work outside of combat.