Quote:

why not use roll or spring pins



Because they probably cost the factory more than a dowel did.

Aftermarket ... when's the last time you saw a roll pin with an offset ?!?!?

In reality, a roll pin could probably be safely used. They're spring steel so they're plenty strong. They are in a "shear" situation so could probably easily take the stress although I don't believe there is any.

If all the bolts on the trans to engine are tight, the trans is held in alignment primarily through the friction of the two mating surfaces - that friction created by the torque of the aforementioned bolts. The pins provide minimal support (if any) once the trans is bolted up.

Argue it all you want, this is my opinion and I'm sticking to it.