What would be a huge improvement is if it would be possible to select guides. This opens up a whole range of possibilities which extensions and internal tools can benefit from. Examples are:

-Create a 3rd guide in the middle of two selected guides.

-Create a 3rd guide equidistant from two other selected guides.

-Select a few guides and delete them. 

While researching this topic, I came across an interesting discussion in 2009 with the title "rotating guides" in the inkscape-devel mailing list, where the interesting bit starts at the 3rd page. It appears that the ability to select guides is a bit controversial.

I would like to express my opinion on some of these matters as I think it is still current. 

I agree that guides are not the same as objects and that not all tools make sense for them. However, I disagree that not implementing a helpful feature because it introduces UI issues is a valid argument. For example, I think that not being able to set the stroke and fill before an object is drawn (except for the color) is unexpected as well. So let's not implement fill and stroke at all, because people might get confused. That is basically what is being said below.

Grayed out dialog items were invented because sometimes things just don't make sense. I think that is the solution to this problem.

Here are some quotes from that discussion:

"if we make guides selectable, this will open a whole can of worms - people will complain that the guides cannot be painted by clicking on a color swatch, cannot be aligned with Align dialog, cannot be grouped etc. etc. A lot of things that are natural for objects make little sense for guides, but by allowing to select them we hint that they are "sort of" objects. The amount of special-casing that this will require in the code and, more importantly, in user documentation and user habits and practices is rather frightening."

"It may be confusing for some people, but I think it shouldn't be for the  vast majority, especially if it's documented.  There are plenty of places where the program can be potentially a LOT more confusing that this, but we don't worry so much if it provides a wanted functionality."