That would make things like a column of phone numbers very gappy, as the figurespace is considerably wider than the space.

Yes it is. But for phone numbers you could also use a thinspace.
The problem is Apps like Photoshop can't access figurespace. Illustrator can but you have to open the glyphs window. It doesn't seem to handy.
On the other way, if an user forgets to deactivates tnum and keep writing, spaces will be huge..
But if we create a tnum version of monetary symbols, why not spaces too.

In which situations would a figurespace mostly be used?

it should be used in tabular numbers, when number groups could have different lenght

to align stuff like here below for instance

234 23
3958 3
1 23 1
     4
   122

or

110-
 29=
----
 81

More about putting non-numeric glyphs in the Tabular feature:

http://typophile.com/node/41191

Thanks Nick, I've already read it, and more times :) but didn't find a clear answer to my Hamletic doubt

Jan, what is that first image you posted?

image? which image? what do you mean? I miss something..sorry

234 etc.

Nick — Not an image, just styled with <pre>. So it probably displays with your browser’s default monospace.

yes exactly :) I used < pre > to obtein a tabular effect

I think Nick was asking of an explanation for why numbers might appear in this particular tabular arrangement.

Thanks John, that was indeed the question.
It's a usage I'm unfamiliar with.

Ok, never mind, I will do not add the sub line to tnum :)
Thank you everybody.

Font Bureau is in the habit of putting substitutions for additional tabular sorts, like figurespace and certain tabular punctuation, in a separate stylistic set so it can be ‘added on’ to {tnum} in situations that require. (And they’ve settled on a standard of {ss20} across their fonts.)

Of course, this doesn’t make the figurespace any more accessible in Photoshop or Illustrator (which sounds like the original motive behind this thread) since these apps don’t support ssets.

Yes no AI nor PS support :( It's a shame both these Apps don't have access to ssxx features in their Opentype panels (at least to v.cs4 which I use actually)