I am tempted to say that if you don't use a metronome regularly in your practicing You're Doing it Wrong, but I'm sure that would be unfair.
Nonetheless, not everyone agrees with me. So here are the top three arguments against using a metronome, and my responses.
Argument 1: It is only for beginners who can't keep time.
My response: Literally no one has a problem "keeping time."
The same beginners who struggle mightily to count to the number 4 in a piano lesson have no problems performing complex dance moves with their friends, or singing along to their favorite songs on the radio. They also have no problems walking, talking, playing video games, or any of the other thousands of tasks that call for highly trained and coordinated senses of rhythm and timing.
The reason they seem to lose all of this in a piano lesson is only because they are distracted. They are trying to do many things at once (play the right notes, read the symbols on the page, please the teacher, prevent themselves from screaming in frustration) and they are caring very much whether they are doing them well. Yes, that messes up your sense of time. The metronome will bring you back into the present.
It is not only beginners who get distracted. Maybe you've learned to hide it, though. You've learned to play reasonably in tempo, despite your distractions, your anxieties, your fears, and your insecurities. Are you paying any price for that?
Argument 2: It is too stressful.
My response: If you think the metronome is stressful, you are probably taking it way too seriously. The metronome can be a calming, soothing force.
Don't try to follow the metronome. That is where the stress comes from. Instead, let it click in the background. Sometimes you're with it, sometimes you're not. It doesn't really matter, does it? Is the metronome going to yell at you if you deviate from its tempo?
If you feel rushed by the metronome, can you let it go?
Argument 3: It leads to mechanical playing.
My response: On the contrary, I believe the metronome is an incredibly useful tool for developing musicality. The purpose of the metronome is not to learn musicality per se, but rather to learn control and poise. Once you have control, you can express the music however you see fit.
It is musicians who cannot play with a metronome, and instead base all of their movements on escaping their own anxieties, who end up playing inflexibly and mechanically.