I had an acoustic guitar track in a song and saved where I was up to as a mix scene.  Cloned the acoustic guitar track and moved the cloned tracks audio by 10 ms and panned the (now two) tracks hard left and right to try and give it some width and then saved it as a new mix scene.
 
Flipping between the two mix scenes didn't seem to change anything.....did I do something wrong?

I'd imagine it would save the pan. Not the clip edit. You'd have to do a "Save As" for editing changes. So even though Haas Effect stuff is a "mixing" technique it is still an editing task (aside from the panning step).

I don't think its ever that simple to divorce editing from the mix process, for me anyway, the two things are completely interlinked. I, perhaps wrongly, assumed that the mix recall feature was more of a global snapshot tool which for me would have been much more useful. 
 
For example part of my "mix" process could well involve playing with vocal sync and rendering tracks but it sounds like that wouldn't be recalled either

However I just thought of a possible workaround.
 
IF MixRecall remembers Mute/Solo states of tracks and takes (I don't know if it does) you could copy the clips into new take lanes, create your Haas effect using the new Takes leaving the originals as is, then for one MixRecall snapshot have those lanes muted with the originals audible and then vice versa.
 
That is of course if MixRecall saves Mute/Solo stakes of lanes/clips and I would be curious to hear if it did or not.

P-Theory
I don't think its every that simple to divorce editing from the mix process, for me anyway, the two things are completely interlinked. I, perhaps wrongly, assumed that the mix recall feature was more of a global snapshot tool which for me would have been much more useful. 
 
For example part of my "mix" process could well involve playing with vocal sync and rendering tracks but it sounds like that wouldn't be recalled either

You are expecting too much from it and in fact that would be VERY undesirable for it's intended purpose which is simply to check mix levels. They could however add an extra option for MR to save edits but I for one would NOT want edits to be saved in my snapshot becauseif I changed an edit I'd want it to STAY changed.
 
Thes types of features are for mixing and based on the assumption that the project is actually in the mixing stage.
 
If you want to retain edits you could easily do what I said earlier and copied clips into new lanes and perform your edits on the new takes.

If you have a look at the options dialog box it tells you what it saves.

If you want to save the track data, ie audio or midi then copy the clip, track, bus, project or whatever.

Mix recall does not include clip editing etc. Neither track or buss removal/addition. It recalls all the 'usual' mixing settings..... Volume, pan, FX, automation etc. etc.
 
If you add or remove tracks and then recall a mix that didn't have those tracks/busses when saved any added tracks/busses are ignored. Much the same for deleted tracks, they are ignored.
 
HTH

Well each to his own, as I said, it sounds like I was expecting too much from it and was hoping for a global snapshot

You could do essentially the same thing using Save As and File > Open. That was basically the old school way of comparing mixes. It would probably only take a couple seconds longer to load.

Thanks I understand now but it does limit its usefulness for me as if it doesn't even track busses and associated rerouting etc then it is a very limited feature for me.  I mean surely setting up things like parallel compression busses is part of a "normal" mixing process?