I'm completely in line with OP, here. There are tons of knee-jerk reactions here about integrity of the product, presentation format, difficulty of maintenance, etc., and they all stem from very valid, respectable concerns. However, as OP has stated, none of those have to be compromised by giving out a presentation API. The interface has little to do with the game mechanics--as any programmer should know, the concerns are (or should be, if Toady is worth his salt) very separate, and encapsulated accordingly. Versioning between interfaces and DF core would be kept separate, and it would be the interface designer's responsibility to keep track of version incompatibility with the DF core.
The RAW file example mentioned by Granite26 is apt--people can already change aspects of the core mechanics by editing those things. Why should they not be able to change presentation, an aspect that has even less to do with the game itself than the RAW files?
Let's consider WoW's experience with a powerful presentation API. Due to the abilities provided to the interface, scripts were, at various points, able to overcome certain WoW design points (for example, an enemy level display on enemies that would normally be invisible). Blizzard has had to introduce (and will indubitably have to introduce) various patches to correct oversights found this way, and has actually used the ideas from the scripting community in its core presentation design (e.g., extra action bars). However, WoW is worried about giving players an equal playing ground in a way that DF is not, because Blizzard has to worry about maintaining a particular persistent world, while DF players all create and share their own worlds. Also, this happens in DF even without the enhanced API, and it has arguably improved the game--consider how 0x517A5D's Regional Prospector (probably) influenced the inclusion of a feature-finding mechanism on the embark screen.
As a final note, I'll say that there's a good chance that Toady's presentation arc will incorporate these ideas, anyway. I seem to remember various comments that indicated that Toady is interested in seeing a good interface for DF, and there's a good chance that opening up the presentation with an API is the easiest way to do that. Until then, there's no need to shout down suggestions on programming design just because they look like an attempt by someone to butt in on the project, especially when they contain very salient points about said design.