Why pants and plots are the same thing
One lesson I've learned from the day job is that if you give one guy three guys' jobs to do he'll subsume those three jobs into his main job. Pretty soon those three jobs become one job. The edges between the disciplines start to blur. Imagine the guy who builds the product now has to design and test it too. He figures only a brief design is required, he can do the rest as he builds it. It's not going to need much testing - he knows it's going to work because he's the guy who's building it. He delivers, and he gets away with it, but something gets lost, something is missing. Before long he's building it without any design or testing at all - and it still works, mainly, and it's still pretty good, mainly. But it's second rate. It's mediocre. It's just 'ok'. And he wonders why.
Maybe you're not like this. Maybe you're disciplined enough to give each aspect of the job the time and attention it requires. Maybe you're not like most people. In which case, you'll make a good pantser.
But the fact is, whether you pants or plot, the ideas that you produce have to undergo the three same things.
Writers like to write and it follows that they like to capture and generate ideas using words. I do it. I have a notebook rammed with ideas from the sublime to the ridiculous. Most of those ideas will never see the light of day and that's a good thing. If you crack on with your manuscript without allowing yourself the time to have those crap ideas, they might just make it to your final draft.
The structure of the incidents, the story, the plot, suspense, narrative drive, believability - if you don't spend some time structuring the ideas and scenes you've invented, then you leave these elements of good fiction to your natural talent – or to chance. Maybe Stephen King can get away without plotting, but can you?
You can have brilliant ideas and a great story, but if you can't express yourself, you're no good to anyone. There's your voice (needs honing), your technique (needs learning), your style (needs choosing) – three things that can in fact benefit by simply getting on and writing – but if you're trying to do that at the same time as coming up with ideas and trying to structure events into something consequential then they may get compromised.
People need to do whatever they need to do to get written what they want to write. Some people develop their characters and outline meticulously before starting to write the narrative and some just start writing – because that's what works for them. Some people do both.
But there's a difference between pantsing and just avoiding the issue – if there's no room for the above three phases of creation in your writing process then you're not really pantsing – you're just typing. It may feel liberating at first – but you're only going to have to fix it later, or settle for being mediocre.