And so, to settle the debate, I asked the people internet about their play styles, specifically in regards to optional level grinding in offline, single-player RPGs. To make sure everyone was on the same page, I (and Wikipedia) defined "level grinding" as "engaging in repetitive tasks (e.g. fighting enemies repeatedly) in order to increase the strength of the player's character or party members".
It's also worth noting that the vast majority of responses were given by users from the /r/SampleSize Reddit community who I asked only to participate if they had completed at least one RPG.
Here are the results:
As for my brethren, the non-grinders, they were also asked for their reasons in the same way:
And, well, that's that.
If you'd like to see all the raw data for yourself, here it is. (Note that additional responses may be recorded after the date of this posting and these were not referenced above, obviously.)
If you'd like to take the survey, you can do it here, though I won't be updating this post with future results.