No choice but to use inserted seats in an aluminum head, and there is this little thing called emissions warrenty. Engine HAS to last 50,000 miles without anything but "normal maintenance" or the producer has to repair it. Otherwise we would still be doing valve jobs at 25,000 miles and a bore and hone at 75,000!
As far as I know, only American engines with cast iron heads still have directly machined valve seats -- everyone else uses aluminum heads.
Besides, did you ever actually SEE an engine that required a valve job simply because the owner used unleaded fuel? I haven't -- unleaded makes the engine last longer!
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!