I don’t think Comic Sans and Papyrus are popular because people prefer them to choices a designer would make. They are popular because they are readily available. Normal people pick a font by looking at the font dropdown in Word. You actually have to pay a lot of money for most fonts on the submission’s list.

A lot of good could be done for this world by operating systems with better default fonts.

Windows and Mac do have better fonts available, so that can't be the sole reason. What both fonts have in common, is that they don't look professional. So they are intended to convey a different mood by their user – which gets somewhat ironic when you consider the amount of passive-agressive notes written in Comic Sans.

Just like deliberate spelling mistakes (flickr, tumblr etc.) are used to convey a certain non-chalance, using "bad" fonts might be used as a tool. It's probably not good for long-standing branding, but for cheap one-off mass market communication, it might fly. Personally, I probably wouldn't, but then again, I'm one of those who considers sans-serif fonts necessary evils in this low-DPI world, not much more.

Comic Sans is often used quite deliberately to give an air of unprofessionalism/cheapness. I suspect that Papyrus is more often used in a misguided attempt to look arty or historic and would probably get significantly less use with better default fonts.

Well, Comic Sans is a poor excuse for a hand-lettered font, and there are a zillion better out there, even if most lay people are unaware of them. Papyrus, though, is a gorgeous font (a modified Carolingian with caps and a distressed effect) that is the righteous choice when used correctly. Like anything that looks simple on the surface, though, people are going to copy the wrong thing when they try to duplicate someone else's success. Papyrus can figure strongly in a successful design, but it's rarely the font alone that makes a design work.

Comic Sans would be an ok choice for comic lettering, you know, what its name kind of implies.

Even then, it's limited to a kind of "cramped technical pen" lettering that would be expected in alternative or underground comics. There are other comic book fonts that would be more broadly applicable.

Anime Ace is a particular favorite of mine for speech bubbles and narrative blocks. Badaboom works well for in-panel sound effects. There are many others, but those two remind me most of my Marvel childhood. (I has Spidey #1 and Hulk #1. They cost a dime at the time, and who the hell knew they'd be worth anything? Same with my Bobby Orr rookie cards -- I traded a bunch of them away for proper Leaf players, and wound up putting the last one in the spokes of my bike the next season.)

Inspired by the lettering for Watchmen, if I remember correctly. Which is why I found the font itself much more palatable, you just have to read everything printed with it in Rohrschach's voice.

Yes, it's supposedly inspired by Watchmen and The Dark Knight Returns, which are both excellent books. However, the letter shapes of those two books are closer to each other than they are to Comic Sans. It seems the font creator relied more on his intuition of how comic letters should look than on observation. There's a good comparison of the three fonts here:

http://kleinletters.com/Blog/?p=3599

There aren’t really any alternatives to Comic Sans or Papyrus on Windows. Everything else looks awful when printed (Verdana) or is, if somewhat more – ehem – professional than Comic Sans, not exactly very emotional (Times New Roman).

They always had Arial. But that is even more formal, as we're used to sans-serif fonts in signage. If you want to inform someone of the fact that there are cookies in the kitchen, you don't want to present it in the visual equivalent of a German border guard.

But never mind what's the original reason, now people are used to these bad fonts. A corner cafe using Papyrus, birthday invitations using Comic Sans… My point is that you could harness that history if you want to advertise your business on the web. Will it be good typography? Nope. Will it sell more than a golden ratio Bodoni layout? Possibly.

Viewers don't recognize the font that something's in. At most, they might recognize the feeling that the font gives them, but that's probably a subconscious reaction as well. Consider the situation in which someone sends an important and serious email, but uses Comic Sans for the text. The same font would work for a birthday party invitation, but since the font doesn't support the context of the communication it comes across as foolish.