Do you ignore whether your HTML is valid just because the browser rendered it correctly?
I've got real work to do. Making a validator happy is fake work.
So, do you want to pay the price upfront when you can plan for it or afterwards when the fix must be done immediately because customers are complaining?
I'd much rather pay the exact price later, than an inflated price now.
Then, working at a startup taught me that it's not black and white. Several quotes come to mind, but Voltaire's is my favorite:
"The perfect is the enemy of the good."
Cowboys may get things "done" quickly, but that doesn't help when things are subtly broken, have interoperability problems, or are nearly impossible to extend without breaking.