The carbon will result in "carbon tracking" where there is enough conductive dust to short the spark around to several plugs at one time. Once it get hot, it burns into place, causing crossfire and misfire.
The rest of the cap looks almost new, unlike the one I took out of the 300TE -- side contacts were almost burned off!
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!