Look at programming languages. There are lot of passionate users, advocating the languages, and things don't turn sour like that. I would even call myself a Lisp and Ruby fanboy, I find it a funny word, not pejorative at all. Add GNU/Linux to that too. (EDIT: Of course I know those things have limitations and I wouldn't use them everything, they're not silver bullets, and that's why I prefer projects where I can use them)
The problem is this "us or them" mentality, when you can't stand people using different things, or think they're making wrong choices.
Just look at articles on ArsTechnica, CNet or specific iOS/Android sites and you'll get a bunch of comments like "iOS users are sheep who like shiny stuff" or "Android users are cheap bastards who don't buy apps".
Being "passionate" clearly isn't the problem here, being an asshole because you're obsessed with some brand, on the other hand...
Harley Davidson certainly recognizes that.
Comic books are the first example that springs to mind, but I guess Harley Davidson might be another good one.
EDIT:
I did not mean to say that being passionate is bad. I meant to say that in most social situations, appearing overly passionate is often bad. Right now, I am struggling to maintain passion for the project I'm working on and I know that I will fail as soon as I stop being passionate about it -- so I know there are hard times when passion is necessary. However I think that my point about appearing "cool" holds -- more people get excited about my projects when I put up an impression of total control and slight lack of passion rather than the other way around.
One has to know how to program both oneself and others.
Of course, you could say Fermat was an outlier, but I suspect that anyone who is truly, deeply passionate about something is an outlier already. Most people go through their lives with little passion at all.
I realized later that unless I become the next Darwin, I will never be happy with my career choice because I will be doomed to working for a large part of my life for people less intelligent than myself and that I will not grow as a person because of that.
Passion is good unless we lose control of it.
Being passionate about a brand though is probably less of a good thing for you personally.
It also means you are attracting nutjobs.
For example, see HN comments on news/opinion involving patents, Google, Android and Apple. They devolve in flamefests because people think that their particular company is being wronged, regardless of their original stance on patents etc. before such news involving their companies came out.
Asymco's analysis always seems to make errors that favor Apple, like leaving out HTC from stats on how profitable Android OEMs are. Or something like this http://news.ycombinator.com/item?id=2887430
Same with Gruber with his extreme pro-Apple spin on everything. Or Marco's 'analysis' http://news.ycombinator.com/item?id=2878329
It's very telling how some geeks find Gruber, Asymco, Marco etc. very insightful, whereas others find them inane and biased to a fault. No wonder the discussion runs into a flamefest with very harsh moderation.
Not singling out Apple here, there are examples where criticizing Google will get you automatically downvoted here on HN.
Some part of the moderation seems to be on 'party lines'. Criticize Apple or Google, or support MS and face downvotes or no votes. Criticize MS, get a boatload of karma. All this regardless of the actual facts on hand.
As you say, all this may be good for companies, especially ones that carefully cultivate their brand image to use such effects positively, but this is not really beneficial to honest discussion, information or analysis. It turns the conversation into Fox News vs. MSNBC. with people unable to admit their favorite company can do something wrong once in a while, resulting in people mudslinging the competition instead.
Also, see Roughly Drafted and Boycott Novell(now techrights.org).
Just because many sources these days are very positive on Apple and its products, I don't think you can necessarily say that they are Apple-biased. The fact is that Apple is killing all of their competitors right now on almost every measure - product design, profit, etc. Insisting on more "even" coverage of their competitors just isn't warranted because in most cases the competing products don't deserve that much attention or consideration.
Gruber, Asymco, and Marco tend to be very positive on Apple, but when the company screws up, they'll be as harsh as anyone else. For a recent example, look at their coverage of the App Store's change in-app purchase and subscription policies .
To paraphrase Steven Colbert, sometimes reality has an Apple bias.