To add to and echo the comments of JCE, it seems there is an element of frustration built in to almost everything. While you’d obviously want to use quality components, it’s entirely possible that this technician was trying to help you, and himself. Albeit in a gruff manner.
Anyway, it’s not that MB OEM parts are necessarily “better;” merely that he has less possibility of wasted time and unexpected problems. He also might be able to return or exchange a defective part, as a courtesy to you, rather than having to have the car occupy his bays and parking lot for extended periods of time while he waits for you to provide a replacement.
FWIW, I’m about to have a 60K mile service done on my 400E. I’m providing the parts (purchased at FastLane) at the somewhat reluctant okay of the technician. I told the technician that if there are any problems, he could just use his parts and I’d return the questionable ones. He was okay with that.
The other side of this coin is that if there is a problem with your independently provided parts, you end up costing the technician in terms of scheduling, lost income, and, back to where I started my comment, a degree of frustration. Frustration in the name of being hindered in his business goals, because he agreed to accommodate your whims. In short, it’s the old story of no good deed going unpunished.
BTW, why did you put new springs in?
'00 ML320 "Casper"
'92 400E "Stella"