Comment 1 for bug 434364

"For instance maybe you could decide to display the sidebar label next to its icon if you consider that the corresponding icon doesn't provide a good enough metaphor (see Midori for a sidebar icon exemple) (but it's not the reason why "priority text" setting exist...)
However there could be some i18n problems : for instance "sidepane" would be translated "panneau latéral" in french and that would make a huge button"

Totem 2.28 toolbar doesn't follow GNOME Style either, but i've noticed that it has a "sidepane" (or maybe just "pane" - i don't run the english version) button with only the label (without icon) whereas other buttons have only an icon.
Developpers may have good reason to go against HIG sometimes : i guess they didn't find any good metaphor for the sidepane and therefore have renounced to display an icon.

I think you have the choice in "text beside icons" mode for the sidebar : displaying only an icon, or an icon plus a label, or only a label

Note that, in french, the (side)pane button is translated into "panneau" (and not "panneau latéral") which avoids to have huge buttons in the toolbar