This kind of view is often not localisation-friendly, as it works best with short names, but names vary from language to language. It can also make even simple interfaces look overwhelming by making them large, which I think is a bad idea for new users - so it should be opt-in. But if it's opt-in, then the people who would benefit from it are unlikely to be using it, since it would require them to find this option. Based on the kinds of questions new users have been asking, it seems most new users do not check the options, or at least don't click things to try them out.
I also think that as a temporary way to learn the icons, it doesn't work, since it changes where everything is visually, so by the time the user is ready to switch to the compact view, they'll have muscle memory for the expanded view, and might as well keep using it. Plus, as a learning tool, it doesn't accomplish anything that mouse-over doesn't accomplish, albeit it does so a little bit faster, and relying on the names rather than the icons might not teach the user the icons at all.
I think small, compact icons are the way to go for an editor like Tiled or Photoshop where there are a lot of icons on screen at once, and they don't change often - the layout just becomes awkward if the names are displayed. On my screen, the Map editor toolbar wouldn't even fit on the screen if there was text under the icons. Text under icons makes sense in programs where there are a few icons on screen at a time, but which change often, as the layout can remain compact enough even with names displayed.
That said, for touchscreens, hovering for tooltips isn't always an option, and an increasing number of people use desktop software on touchscreens, so a mode for that could be beneficial. However, I think it'll take more than just showing the text underneath - it'll probably require alternate, brief tool names, and possibly a different style of toolbar (or at least toolbar customisation, as mentioned by bjorn), as well as, ironically, more toolbar toggles to replace the need for modifier keys.