Last time I rushed a glass cut, I turned a perfect pane into a puzzle. @lopez.linda is right about new tools making a difference, but my hands still have the old habits.
My old boss at the glass shop in San Antonio had the same argument with his brother all the time. I'm totally with you on the new tools thing - we switched to diamond blades last year and it changed everything. I remember one time we had to cut 20 panels for a storefront, and doing it fast with the new saw saved us hours (and no cracks, believe it or not). Your uncle probably learned on the old equipment, so his caution makes sense, but tech has moved on. Now I wouldn't go back to slow cutting even if you paid me.