Hardend valve seats make both seat and valve last considerably longer than "plain" valve seats ground into a cast iron head.
Part of emission certification includes long term operation of the engine to certify that the design will meet emission requirements at extended milage (or you can guess what they would run like in 15,000 miles!), and "soft" valve seats will be leaking enough at 50,000 that compression is way low. Low compression = high CO/HC, so there you are.
Even hardened seats and stellited valves wont' last forever -- the compression in "old blue" at work (92 Chevy V6 van with 350,000 miles) is down to about half spec -- doesn't use oil, so I suspect worn out valves. However, new valves would probably result in a blown rod or crank is short order -- I believe the only reason it still runs is that the power output is so low.
Besides, do you really think GM or Ford could sell a car in this day and age that required a valve job at 25,000 miles? Get real! Most people I know get exercised at the thought of brake work at 25,000 miles, let along the thought of engine repairs.
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!