That is exactly the point.
It's totally useless to put something else next to a mex.
You think it's ok to put a fac next to a mex...it's not.
For example:
One Loyalist with a basic Fac
12 mass/sec
With T2 Massfabs
10.8 mass/sec
With T3 Massfabs
8.4 mass/sec (-30%)
Next to a T1 mex:11.7 mass/sec (-2.5%)
Next to a T2 mex:11.55 mass/sec (-3,75%)
Next to a T3 mex11.4 mass/sec (-5%)
And those examples are done with an expensive unit.
Think it again with mantises...2.5% ??? After 5 minutes it's something like 1 free mantis...what a tremendous advantage.
Not to forget that adjency is not given to assisting engys (if you upgrade the fac to T2 or T3)
Is it viable? No
Is it good to give the possibility to do it? I think it's not.
I've done the math 3 years ago (
http://supkek.blogspot.com/2009/02/bonus.html) and 90% of the adjency bonus are crap.
it's useless and should be redone one way or another.
For example I think the T2 mex should be able to build the storage by itself.
Like the 1button upgrade for T2 and T3, we can put another upgrade "build storage" or put an additional 2.5 upgrade step.
It easier for noobs and pros can do it the old (and better) way with engies.