As a user & wanna-be dev, I'll chime in. =)
First off I'll state that I use inkscape for everything from doing design work for the law firm I work at to my personal use which is purely illustration, so I have some varying/diverse needs.
I have found only one situation where userspace/bbox makes a difference: Paste style. Consider a rect with a vertical linear gradient, black to white. Copy it and paste the style (Ctrl+Shift+V) to another rect which is below the first one. If the coords were bbox, then the new rect will also be black-to white in its place. If the coords were userspace, however, the new rect will be all white, and its black-to-white transition will be outside of its shape, in line with the old rect above it.
As for me, I do not consider this a disadvantage, because I often need exactly this behavior (i.e. I want the gradient to stay unmoved relative to the canvas so that the new objects "continue" it) and rarely if ever need the other behavior (where pasted gradient is moved to the position of the new object). Others however may have different preferences. There may also exist advantages to bbox coords that I'm missing. Hence this call for opinions; please put forward your thoughts on this. If there's no opposition, I'll proceed with the above plan.
Ok... so first off I have a Q. In the stroke & fill editor, when working with radial gradients, it has a dropdown box for "coordinates" where you can choose object or user space. Would this still work the same since all coords will be userspace? I can tell you I use both userspace and object for that option frequently. If it impacts this function, I say it would hurt too much. But, I'll assume this is unaffected as you state the pasting of styles is the only thing it should affect.
However, more times than not, I need to paste a style that will be the same as what was copied, not somewhere else further down along the "line" of the gradient. Primarily with layout/design stuff I need it to be the same. And same goes for a significant chunk of my illustration stuff... although I could definitely see it working to my advantage to have that continuation sometimes (had about 10 examples pop into my head). So if you do decide to go ahead and make that change, I think I'll file an RFE for more controls over copying/pasting object styles (if the behavior could even be modified once changed).
Hopefully that makes sense (and isn't dumb)... -Josh
PS: In case you're wondering what type of illustrations I do, one of my better (yet far from complete) examples is at http://www.deviantart.com/view/10029092/. And sorry for link to a raster image, but my personal art isn't open source. =)