Not always the case.I regularly use my watches in and under water, and I'd far rather have a 10-12mm diver. Despite a technical diving background, I don't need 300m WR.
And I'm glad you highlighted the downside (undue case thickness) of high WR.
The core "Sub" dive watches by NTH have 300m WR and are 11.5mm thick.
The <40mm Breitling Superocean dive watches have 500m WR at 11.2mm thickness.