I think once you put the word Serious in the header like you did you triggered a kneejerk reaction to a buzzword that conjures up class division. "Serious" implies wealth when it precedes words like Investor or Collector. It conjures up the impression that you can only be Serious when you are throwing big money at something. Society has ingrained that into our collective psyche, that there are the Serious Big Boys, and then there's the rest of us. Naturally the rest of us tend to loathe and react negatively to the Big Boys when a comment is made that dismisses the rest of us as secondary and not "Serious" because we are not throwing big money around and calling the end result "Better Than". Your intent was innocent but your choice of phrasing hit on a negative trigger. Perhaps a better choice would have been "Who takes their collecting seriously?".
I guess anytime you begin buying more than one pipe of a certain genre, from a certain maker or of a certain style you begin collecting with a somewhat serious intent. I have collected several genres of the pipe world, including classic British pipes, Italian high grades, Danish masters, and some classic pedestrian makers like Peterson. Each time I chose to collect a few pipes from a genre I took it seriously and did my homework, even though I may have ended up only gathering a few pieces before moving on to something else. So I guess, because of my intent and the manner in which I went about it, I am a serious collector.
I know of serious collectors who specialize in Dr Grabows and Kaywoodies, and I know of serious collectors who buy pipes carved by Sixten and Teddy and Lars. I think the more serious collector is the guy who, regardless of the pipes status or monetary value, puts more thought and effort into his pursuit rather just buying it because it is what it is. Some collectors take great care to preserve their pipes in the best condition possible and some just smoke em up, charring up rims and burning through walls regardless of the cost of the pipe. The more serious collector may be a rich man or he may be a poor man, but I think it matters less "what" he collects, and more "how" he collects.