Why T2 connector with a binoviewer? Sorry for the off question, but I want to learn
If you are asking why everyone is interested in getting T2 compatible adapters for their WO, Denk, and other kinds of binoviewers, it is becuase by eliminating the eyepiece holder and coupling the binoviewer directly to the top of the diagonal.
As you have surmized, this is of particular value to people using SCTs and refractors.
In the SCT, you get about a 30mm focal lenght reduction for every 10mm of light path you can reduce.
This means if you go from a 2" diagonal to a T2 diagonal, you reduce the light path by about 30mm, which reduces the focal lenght by about 90mm. If you than eliminate the eyepecie holder, you save maybe another 60mm of light path.
This means that for a C8, by moving from a 2" traditional diagonal to a Baader Prism and using an adapter to connect the binoviewer directly to the top of the diagonal, you can save as much a 200mm in focal lenght, and in a C8, this would give you a 10% increase in true field size.
In a refractor the benefit is also in true field.
Suppose you need a 2x Barlow to reach focus when using a 2" diagonal. By going to the Baader diagonal, you might be able to use the 1.7GPC to reach focus. Then if you remove the eyepeice holder and direct connect, you might be able to move to the 1.25x GPC! That would resulte in a huge gain of true field.
So, the shorter you make the light path, the more potential for bigger, brighter fields.
Why the Baader? Because they are the only one that produces very short light path diagonals geared up this way.
There are other approaches. Denkmeier has a very short light path 2" diagonal that allows the attachement of the binoviewer using their own dovetail system (I think it is based on WO diagonals) and this is a great compromise for someone that wants to still use 2" eyepeices.
It would not have to be T2, but it just happens that the Baader stuff has been around a long time and they started with the T2 theads.
For a Dob, there is still some benefit. Using a T2 connection, it would be possible to mate a WO or Denk to the Baader 1.7x Newtonian Coma Correcting Glass Path Corrector. This would give much lower power than a Barlow.
The unit is expensive, but it does allow for a way to get lower power and wider field in a Newt.
But by the time you added the Newt GPC, you might be better off just selling a WO and buying a used Denk II.
Anyway, the Baader T2 stuff just adds a huge amount of flexibility, and shortening the light path in SCTs and Refractors usually has multiple benefits.