Major Engine Problem - Seeking Answers

New to the forum, but will be brief.
I own 2003 C230 kompressor. Got it passed July with 42000 mi on it. Since then I put on it over 8000 mi.
Two weeks ago I had exhaust valve springs replaced. Dealer said that the engine light on and rough idling is due to a carbon buildup on the exhaust valve. Week later I went back to the dealer with the same symptomps. Today dealer told me they need to replace whole cylinder heads along with all valves, sleeves and springs replaced again. In one word they are planing to rebuild whole top part of the engine.

The sytuation makes me uneasy.

Souldn't they replace pistons while they are at it?
Or if it's such a big deal, that - according to the service guy - "Mercedes told them to replace this, this and that" - they should put in new engine?

This car is certified and has extended warranty. So far all repairs are paid for.
I am so close to get back to the dealer and negotiate a replacement for this car. I know Mercedes, or at least the old, good quality of it. They were never disposable. At least not after 50000 mi.

What do you think?

if the engine is burning oil, the piston ring must be put into consideration as well, or else the same problem might happen again.

thx, but I always thought that carbon buildup is related to crappy fuel quality, not oil burning.

if your fuel got a lot of tar or sugar on it, then what you thought is right too.

here's what i just found out about problems i've been having with mine with 68,000 miles http://www.mercedesforum.com/m_34290/tm.htm