I can't answer for BBC, but here are my reasons:
(1) because of the spill-over to other threads: in times in which there are many BS threads of that type the tone in the music threads also tends to get worse.
(2) because of the bandwidth: I have contributed to make the new super search possible and fear that in a few months this search will be down again; it is getting increasingly difficult for me to find the information I come for.
(3) because of the impact on casual (or not so casual) visitors: some people I'd like to frequent this site are repulsed by these threads and others I'd not particularly like to come here are attracted. (that's a guess from me, not knowledge)
So my reasons are not these threads as such (you're right, I do not need to open them or at least not read them) but their impact on the whole Mudcat 'atmosphere'. There are many threads I do not open for lack of interest or knowledge (e.g., fingerpicking), but have not the slightest objection to them being here.
BBC, thanks for starting this. I was near to start an abundance of 'why are Germans so...', 'why are Irish so...', 'why are Italians so...', 'why are Australians so...' threads and to insert the respective worst of prejudices, but I feared someone would take this too serious and start posting to them true to my words and not to the intention.