Because Ian Hickson use twisted plot and exception to make a point he has already decided on his mind before studying the subject?
Yes, I said _most_ authors. If you are one of the few authors who understands how to avoid the issues raised in this document and does validate all their markup, then this document probably does not apply to you
Ok fine, then why bother us with it? We know what we are doing, thank you very much, go back to bed.
Also note that I would personally suggest that even advanced authors not use XHTML sent as text/html, since many authors copy and paste markup from others and thus may easily end up copying the valid XHTML markup but using it as HTML4.
Fine, make it a personal recommendation. Do not write and advertise such a personal, weightless comment, and title it ?Sending XHTML as text/html Considered Harmful?. Considered implies some kind of authority, you don't have it (yup, being paid by Google to work on HTML5 doesn't go with an ?boss hat?).
Myself, I make the opposite personal recommendation. XHTML 1 strict (sent as text/html or not, doesn't matter) is easier to validate, so it's better for newbies (people who don't validate their html/xhtml are not newbies, there are either idiots or morons). It's also compatible with xml, which make it easier to integrate with advanced tool, and probably the future (?probably? because HTML5 took a different turn, but we'll see).
Mainly, it's an opposition view thing (and the fact that the document linked by you shouldn't be put online without a strong context). He thinks (being a WHATWG member does that to you) that we should improve HTML. I think the whole thing is flawed from the start (despite its incredible achievements) and will never have proper support from UA vendor, and we should move to something else entirely (xml was a nice candidate, however I don't have preferences) that will enforce validation and proper specs.
And to be honest, this is a über-anal-geek discussion. On a basic level, for everyday use, HTML or XHTML are exactly the same thing. So it doesn't matter for 99.999999% of its user that PunBB is written in XHTML or HTML, as long as it uses strict doctype and validation.