Just curious, in 7.5.9 what keeps the columns from resizing like body_col does?
I understand exactly what you're asking, and want to expand your question to also include many items related to a mobile layout, since this is what the main topic of the thread is titled as.
A:
1) Your style's custom width setting.
2) Inlined widths within the right/left columns.
3) Hardcoded settings within the ubbthreads.php library.
4) No @media queries within the reset/common css file.
5) No header viewport scaling settings.
Bonus A:
1) No javascript to toggle the column display back ON for mobile users on sites which have navigation information within their columns.
2) Several forum elements are wider than a 320px screen, or wont allow wrapping. A post's Edit/Reply/Quote button line pushes the page out of screen bounds on devices such as all iPhones and older, non-flagship smartphones.
3) The navbar wraps in the middle of menu items.
There could be more, but these few are just off the top of my head
