For what its worth, in my experience....

Glow plugs will fail suddenly (one day it starts, the next it doesnt), so every diesel owner should have a few extras on hand.

Also, in my experience the glow plugs will also lose their effectiveness with age.

All else being equal (assume good starter, new battery) try starting a 123 diesel when it is zero deg. F with 3 year old glow plugs, then try with new glow plugs...I believe you will have much better cold weather starts with new glow plugs. Yes, carbon buildup may be a factor, but when I take out old glow plugs, I don't see so much carbon to effect the function of the plugs...if the engine is running well, (not burning oil, etc.) then carbon buildup on glow plugs should not be a big factor.

Also, because in cold climates you will occasionally have to crank the engine for an extended period (10 seconds or more) in order to start the car, you also need a good battery in these cars...not one that will fail after the first or second try.

For the above reasons, being in New England, I replace my battery and glow plugs every three years, even if they have not failed. That comes to about $35 a year for a good battery, and $20 a year for good glow plugs...well worth it to me. In warmer climates, I dont think this would be necessary.

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)