It's Actually Pretty Simple
You probably had enough of this whole "designer should know how to code" crap. But I would like to jump in the pool and give my quick opinion. If you're bored of this topic already, here's a fat lady eating something.
Now that we have that behind us, the way I see it is simple. It's not like you HAVE to know how to write mark-up. Sure I know plenty of designers that can't. But is that the optimal situation? Nope. It's not. Out of all my friends that do not know how to write mark-up, I am pretty sure there is not one that doesn't wish they had the time to learn how.
I think what my peers are trying to say by suggesting designers should learn how to "code" is that without knowing how your designs are built it's hard to design for an optimal solution. For example, if I am to design a business card, the optimal design would be achieved by me knowing which paper and printing method will be used for this card. To help you visualize this, here's an example: if I design a letterpress business card, I would probably keep it simple and not go crazy with colors, because the effect of the letterpress should be part of the design and besides, every additional color is pretty expensive. The fact that I am aware of these details, informs my design and will probably lead to a better design.
The same applies with web design. For example, because I know that it's really hard creating an ellipsis in a paragraph according to the height of a containing element, I would probably avoid that in a simple design. Sure, my friend Joe made it happen, but he is a serious developer. I may just use that plug in if I really need to make that happen, but if it's not a must, it might be best to avoid this particular design solution. For sure it's something I should be aware of, before I start designing.
It's not that my peers are pimping the idea that designers should know how to "code" because it's the cool thing to do. Heck, if the standards movement didn't exist, and Flash would be the default, I would probably tell you screw HTML, you need to know a little Action Script. Since HTML is what drives the web right now, and you, as a designer, are working within this domain, you should know how to work with HTML, even at the most basic of levels. Obviously, the more you know, the more your designs are informed.
Start somewhere, trust me, your designs will only improve.
Now, back to your regular programing schedule.