illustration (attribution, if any possible, is at the end of the article)
Mindfulness is only one spoke of the "eightfold path"; the 'observer' stance is useful as a component of the way of life free from biases and ignorance, but this way of life isn't reduced to solely an equanimous focus on whatever object you pick.
For a start, what makes you pick this particular object of mindfulness? A tendency, a habit, a contextless view (i.e. a prejudice), an emotional reaction to its arising? This 'motivation' may also be an object of mindfulness!
Some meditators are able to be equanimous to an object (not letting like/dislike bias their response), but not to the 'arising' itself of the object: 'what' disturbs them doesn't matter, but they still experience an automatic repulsion at the disturbance. This is common when meditation is associated with e.g. the idea of 'calm'.
The value of observation is in the information gathered (meditation is a tool to help insights), information that allows an 'appropriate' response. Mindfulness is key to developing an observation free from biases, preferences, prejudices… but its value still lies in what it later allows to function in accordance with what's needed and constructive.
A 'classic' object of mindfulness (for training purposes) is the breath, and many people become 'experts' at observing their breath without interacting with it, engaging with it, judging it… That's a great skill! But if the breath is inappropriate for the context at hand, is it really 'wise' to just let go on like this? Sure, you don't want to start some sort of emotional panic about 'your' possible chocking (suddenly made poignant solely because it's 'yours' —while continuing to blissfully ignore that people die around you every moment). But, without panicking, without views about a 'self', without views about a 'permanent' contextless way to breath 'correctly', maybe some 'right' action is called for nonetheless?!?
Another 'classic' object of mindfulness (for training purposes) is knee pain. To an extent, it's a great exercise not to buy into the "I dislike; let's see if I can rearrange the circumstances to my liking" samsaric ignorance of reality. But at some point, it just isn't appropriate or wise; it becomes a case of prejudiced stubborn clinging to a particular preconceived posture. Without panicking, maybe some 'right' action is required nonetheless?!?
The way to practice mindfulness and concentration is like a cat waiting for another playful little one to jump… There's no need for anticipation, for pre-committed response, for prejudice, for certainties, and no need for "preventive strike": the little one is let free to change its intention, options are not taken away… There's no need for endless narratives "if this, then I'll do that"… But should playful junior actually jump, the response is swift, without doubt (by pondering endless scenarios), and right on target!
Observing 'freely' doesn't mean suppressing all responses, only suppressing the 'automatic', conditioned reactions tainted by preferences and anticipations. Not falling into the net of distractions doesn't mean fleeing or developing aversion towards action. "Right speech" and "right action" are not contradictory with "right mindfulness" and "right concentration", and we cultivate the spokes in parallel, not one after another!
Imagine you're practising some form of meditation, and you hear screams for "help" in the street.
The practice is not to sit there mindfully: "hearing… hearing…" This would not be developing equanimity or insights in the nature of reality, this would rather be cultivating self-based clinging (" 'my' practice is what's most important right now"), selfish aversion ("why do they disturb me? Can't they attack each other at another time or somewhere else?") and fear ("I don't know how to fight, I'll just get hurt: there's nothing I can do").
The practice is to respond wholesomely and, in such a situation, it's likely your meditation can wait. It's also likely that you don't need to fight to intervene, to do good, or simply to call the police and an ambulance right now rather than count on 'others' to do so (while they tell themselves similar stories and count on you!).
If this seems an extreme scenario, then 1. it happened so don't assume it cannot happen, 2. adequateness with the present causal web matters, wether you call the situation at hand 'exceptional' or 'normal' is irrelevant. Right mindfulness is not reducible to "avoid right action as much as you can, and only switch attitude in the most 'extreme' cases".
With neither avoidance of the situation (turning a blind eye, pretending it's not happening) nor obsession with it (losing one's composure, stopping meditation at each and every tiniest stimulus), there's no need for anticipation, scheming, pre-committed response, prejudice, certainties… no need for "preventive strike" either: the little one is let free to change its intention… the criminal is let free to stop (and even amend… cf. Angulimala (MN 86)). There's no need for endless narratives "if this, then I'll do that", no need for stories 'justifying' the cause or the effect, and no need for endless retaliation or revenge… But should playful junior actually jump, the response is swift, without doubt, and right on target! What needs to be done needs to be done: one doesn't hurt junior, but that doesn't mean junior is left unanswered if it needs answering. You can remain mindful of your body, thoughts, intentions, while intervening! Don't confuse 'posture' with 'mindfulness' itself!
"Help! Help!" is not just a laugh; it's selfish not to respond (whether it comes from the street, or somewhere else…), and delusional to pretend that 'formal' meditation always is the wisest, best way to promote non-violence or the cessation of greed and hatred! There are three ways, and you're responsible for picking the most appropriate one at each moment:
« To do no evil,
To cultivate good,
To purify one's mind:
This is the teaching of the Buddhas. »
MN 86: accesstoinsight.org/tipitaka/mn/mn.086.than.html