Just an FYI that a few of us "downstairs dwellers" have discovered regarding X3 and the new WAVES Reel ADT plug.  It seems that:
Because of the specific mixer architecture in Sonar, It appears there is actually no use for the Mono->Stereo component under Sonar.
If you need a Mono to Stereo effect under Sonar, load the Stereo component on a track with a Mono audio file playing.
 
This is according to WAVES tech support.  I haven't a clue what the mystery "mixer architecture" is...and would very much like to know, so hip me if you do.  But I do know that the mono to stereo, while providing the effect, does not let you pan the source and the adt as you should be able to.  No biggie.  Maybe one of the Bakers can weigh in on this.  Noel??

I think what they mean is that SONAR doesn't allow having different input and output plugin interleave which is true.
We don't allow setting a plugin to have a mono input and stereo output currently in an effects bin so its always symmetric, as a result that plugin feature is not available. TBH I think plugin interleave is complicated enough without having to deal with mixing and matching input interleaves :)

So we should just use stereo plugs if we want stereo effects and never use any mono to stereo versions of ANY plug??

I think most mono to stereo versions support both mono->stereo and mono->mono so that can be used in theory just not to do mono-stereo today. We do support mixing and matching plugins with different interleaves within a bin however, so you can have a stereo plugin followed by a mono plugin. 
How important is it to your workflow to have plugins with mono input and stereo outputs? This is not something that we have supported in the past since its wasn't a common enough scenario.

Well, I guess my question in answer to your question would be:  Why the heck do they make such things in the first place? I have a mono vocal.  I want to use ADT with it which, for the full effect, produces a stereo output.  I would logically think what I should use is the mono to stereo version.  Or would I?  Quite frankly, lots of my way too large plugin collections offerings have mono to stereo versions.  My assumption is there's a valid and ubiquitous need and/or usage for such.  Could I be wrong???  It wouldn't be the first time;-)
 

I wasn't saying you were wrong - I know the use case. I was trying to gauge how important and useful this particular capability is to your workflow. i.e. there are lots of nice to have features but some are more used than others.

Well, in this case it's fairly pointless as I can simply instantiate a full stereo version of the plug.  So again my question becomes:  Why use mono to stereo at all...for any plugin??
 

Interesting...   I don't know if I even have any mono to stereo plugins, but now I'm curious as to the feature or audible differences (if any) between mono>stereo and stereo>stereo versions of the same plugin when used on a mono clip.
 

It seems like it would be fairly common to take a mono input (a vocal or guitar, perhaps) and create a stereo output (delay, reverb, chorus, etc.). Would you just use a stereo plugin? I'm confused now, because I thought I had been doing this for years.Does the Waves ADT plugin work on a mono channel? I haven't tried it yet.
 

Right there are some plugins that take a single channel mono input and can produce a stereo out from it. Like a reverb or guitar processor. Although you could argue that if you gave it a stereo input with 2 mono channels it should be equivalent...