My 2007 335i behaved the same way. There were various explanations about it allowing the engine to warm up better, and there was a "hack" to open both of them up all the time. I tried the hack and it might have had a bit more growl at low rpms, but overall it wasn't a big deal. As for the tailpipes, both of them got equally dirty on my 335i despite one not being open quite as often.