So does BMW. Now go to the BMW forums and see how many people are having sludge problems with their engines. You'll find a large disturbing number.
I've owned an '07 335i since new and hang out on bimmerfest.com and e90post frequently.. This is not true. Oil sludge is not a recurring issue. If you were talking about carbon build-up on intake valves (due to direct injection), dying electric water pumps or oil leaks from bad valve cover gaskets, then I'd agree with you.
BMWs use "condition-based service" for oil changes. Up until this year, that generally meant about 15,000 miles but not necessarily always. The computer tells you when to bring it in. Synthetic oil is mandatory. FWIW, I understand that for MY2014, they increased the oil service frequency to about 10K, so the gods in Munich do apparently see some economic benefit in more frequent changes.
FWIW, I've always changed my 335i oil at approx. 7,500mi intervals, or about 2X as often as the factory recommended. The previous owner of my 4runner changed the oil every 5K like clockwork until I bought it with 95K on the clock. The last time I changed it, I put in Mobil 1. I don't plan to change it until 7,500mi have gone by. The 2UZ-FE is a pretty "lazy" engine compared to a BMW N54 and I don't think it's stressed much. YMMV.