|This post is only tangentially related to Arc.|
If you started learning web development from online tutorials like I did, you probably noticed the general consensus from top search results that table layouts are sin. "Your content and presentation should never mix!" they declare.
I tried to be a good student and take this lesson to heart, but creating nice layouts in CSS is really hard. There are so many ways to alter the position of elements, from floats and clears to relative and absolute positioning, changing display properties between block and inline and inline-block, etc. I have trouble keeping them all straight, let alone how they interact with one another and their respective browser bugs.
Recently a time-sensitive project didn't permit me to fumble any longer with these properties to try and get the layout right, and I was desperate for an alternative. Reading a compelling essay  and trying a few examples finally put me over the edge: I do my layouts in tables now and doubt I'll be going back soon.
Of course, if my CSS skills were better, I'm sure I could figure out how to create solid layouts from CSS properties. (I know that people do this, and I have respect for them.) And of course, I could use a grid CSS framework like Blueprint, though tables are working so well atm, I'm not really sure what the advantage of this would be except to say that my layout was done in CSS.
(Note that I'm only advocating using tables for layout. For most other styling, CSS feels superior to the early presentational HTML tags and attributes.)
So, for any other budding web devs out there, I'd recommend not ruling out tables for layout as the top-ranking tutorials would so readily have you do. And even if you decide not to integrate tables into your standard templates and idioms, at least consider them part of your toolbox.
The relevance this has to Arc is html.arc. I'd previously considered a lot of the utilities in html.arc too dated for web development in 2011, in part because of the emphasis on tables. While I still might not use a lot of the other presentational HTML elements included, I wouldn't hesitate to take advantage of the table macros.