A strong Deutsche Mark would make Germany's exports more expensive, last thing they want (see what's happening to Switzerland right now for example).
Germany’s strong exports are being bought by all European countries which some argue is the cause of the rest of Europe’s unemployment and fiscal problems. Us in the USA like to think it is because of bloated entitlement programs, which very well could be the major cause or a contributing factor, but it is at least plausible that the absolute real cause is Germany’s strong industrial position combined with their ability to export to the rest of Europe.
If Germany left the Euro, their currency would be so strong against the rest of Europe that it would vastly damage their competitive position in Europe leading to a massive transfer in wealth out of Germany.
I seem to remember reading an article around 2007 or so (anyway, before the shit started hitting the fan) about how the German employees' real wages had been practically stagnant in the previous decade, and about how the employers' association had basically told the unions that "it's either you keep your wage demands in check or the jobs will move to Eastern Europe". It seems to have had worked, if only for the fact that the German consumers weren't as easy going with their money.
One of the main problems of that time was changing currency values. The problem was not that the Deutsche Mark was high or low valued, it was more like the change was a problem. That made it difficult for companies to export. Large exporters were insuring themselves against currency value changes.
Basically a high valued currency made it cheap to import goods, energy and materials. A low valued currency makes these imports expensive. For example Germany imports most of its primary energy sources like oil. Devalue the currency makes the oil, gas and coal imports more expensive.
That the German industry depends or profits on a low-value currency is a myth. The Deutsche Mark was never low-value - just the opposite. Still exports were similar like they are today. The German exports will adapt.
The nice thing of the Euro is that the largest market for Germany is right around it and much of it is traded in Euros. This makes trading on variable currency values obsolete. This is the big win for German companies: it makes trading much easier and much less costly.
Second, a somewhat stronger currency would be the lesser evil for Germany than the alternative, which is being robbed blind which is what's happening at the moment.