True, but everyone's idea of a "well-fitting shirt" is different. Some like them fairly form fitting, others prefer more room in various places etc. Just look at the incredible variety in the fits posted on WAYWT. If you simply give an MTM operation your body measurements, you're leaving it completely up to their interpretation, other than using imprecise words like "slim fit". If you take a shirt to a tailor, you can pretty much get it to fit how you would like.
But if you can't be bothered, then its definitely better than just guessing.
I suggest that if you don't have well fitting shirts to start with you won't know how you want them to fit either and furthermore what you want may not be ideal / the most flattering*. Using darted waist measurements can cause problems as the cloth is only being take out of the back**.
*As a general comment on people in this situation rather than advice about this situation or posters.
** You may or may not need/ want darts in the final shirt anyway.
Jmills about a 3.5, so a narrowish spread I suppose. (Credit Kabbaz for the image)