(http://paulgraham.com/identity.html talks about this some, I'm sure there are other better references I can't find right now.)
The problem is one of identity. We will protect anything that we attach to our identity because we perceive any attack on it as an attack on us. To have intelligent, unemotional conversations that has to be decoupled.
In general it can lead to problems if we map our identity externally - whether to a programming language, or something else - of course this is easier to preach than to practice.
(What do programming languages, operating systems, and text editors all have in common? They're big parts of people's identities...)
The boundary of our self is constantly spreading outwards onto the things around us. Seems to be part of human nature.
By itself, self-complexity is neither good nor bad, but it does have consequences. High self-complexity buffers you against negative events or negative appraisals of those aspects you identify with. Someone who's devoted their life to low-level kernel hacking is going to take it more personally when you say C is obsolete and only a fool would be involved in OS design in 2013. Someone who also sees themselves as a husband and a father and a good friend and a church leader and not all that bad at Javascript web programming either is probably going to let it roll off them; they may think you're wrong, but they'll just shrug and say "Whatever; you're entitled to your opinion" and not bother to argue the point.
So no, it's not bad for new activities to become part of your identity. It can be bad for them to become your whole identity, because it leaves you really vulnerable to outside attacks on your self-conception.
(On a side note, it seems to me that a lot of the Silicon Valley startup mythology is focused on encouraging low self-complexity and an obsessive focus on external success. Now that I re-read some of PG's early essays, several of them seem actively harmful to one's mental health. The YC application used to ask you "How are you an 'animal'?", in reference to an early essay where he suggested that successful startup founders often act like caged animals - as if denying your humanity is "success".)
It's people who work at normal jobs that he calls caged animals, and founders are the wild animals.
"In fact, getting a normal job may actually make you less able to start a startup, by turning you into a tame animal who thinks he needs an office to work in and a product manager to tell him what software to write."
http://www.paulgraham.com/start.html
"One of the best tricks I learned during our startup was a rule for deciding who to hire. Could you describe the person as an animal? It might be hard to translate that into another language, but I think everyone in the US knows what it means. It means someone who takes their work a little too seriously; someone who does what they do so well that they pass right through professional and cross over into obsessive."
It used to be of course that you were a Ford man or a Chrysler man. But nowadays it's become much rarer.
I agree that it's natural, but I think that the targets for identification shift over time and I don't think it's random.
Once you identify with something it becomes difficult to be objective about it. So discourse about technical tools is mostly emotion, however rational it pretends to be. That's why the core debates never end.
By the way, you can add version control systems to your list.
Programmers are sometimes maximizers rather than satisficers for this kind of thing. We don't want to know if something is ok and will get the job done, we want the best one.
I think people argue for their language out of some sense of the network effects involved: if I can get more people using my language, there will be more libraries, more people to ask about problems, more people to hire/to hire me, more books, and so on and so forth.
So I think there is some economic rationality to it.