Deference-all-the-time is an incredibly convenient fig leaf for 'take whatever abuse I feel like subjecting you to'.
In a healthy relationship, the deference isn't abuse. In an unhealthy relationship, deference enables abuse.
Edit: Additionally, decent people can end up poisoning a relationship, if they never get any negative feedback in response to their poor behaviour. (Or if they completely disregard any negative feedback, because their opinion is the only one that must be deferred to.)
In a healthy relationship, the deference isn't abuse. In an unhealthy relationship, deference enables abuse.
Edit: Additionally, decent people can end up poisoning a relationship, if they never get any negative feedback in response to their poor behaviour. (Or if they completely disregard any negative feedback, because their opinion is the only one that must be deferred to.)