There is a tendency for any diesel engine block not to give up old glow plugs easily. Look at any glow plug....its pencil thin compared to a gas spark plug.
The amount of torque when trying to back them out is alot of stress on the thin steel threaded barrel. Anti-seize is a very good thing......especially to prevent electrolisis(sp?) between the glow plug steel and the steel alloy or alluminum alloy heads which locks the threads in place like a bad weld.

Even the 6.5 litre TD engines from GM had a bad rap for seized, broken and even "mushroomed" glow plugs.....but I never had a problem with mine in 375,000 miles. And never had a problem with my other two past MBs

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FRED

Daily Driver: 98 E300TD 199K
Hobby Car: 69 Austin Mini
Past Diesels: 84 300SD, 312K
87 300SDL, 251K
94 Chev. K-1500 6.5Ltr.TD, 373K