Revisiting, as I'm installing an engine and gearbox and thought of this discussion.
I, too, wondered why the 'overengineering.' But, after a lot of thought it occurred to me that the purpose of the dowel bolts isn't just to make mating up a gearbox to an engine a frustrating task, or to spend money unnecessarily, but to solidify--can't think of a better word--the whole drivetrain. IOW, when the engine produces torque, it wants to twist against the gearbox, and without the dowel bolts, or similar mechanism, the block, rear engine plate and bellhousing would work (twist) against each other. I suspect that is why there are dowel pins in the rear engine plate/block--can't remember where they are--and dowel bolts between the engine plate and the bellhousing; without them the three parts could twist in relation to each other when torque is applied from engine to rear wheels. IOW, you have three flat surfaces--block, engine plate, and bellhousing/tranmission--that are being encouraged to slide against each other, and the dowel bolts are there to mitigate that movement. It may not make a huge difference, but the dowel bolts would prevent or minimize any fretting in the (relatively) oversized holes of the plate and gearbox. Since the front engine plate does not torque against another component, they are unnecessary there.
That's my theory, at least, and it makes sense to me; if someone knows the canonical truth please correct me. And, you'll say, 'Why don't the bellhousing and gearbox have dowel bolts or pins,' and my guess would be that that the relatively small surface of the area, relatively large number of bolts, and protuberances from the gearbox eliminate the need. Again, just a guess.
And, there's something to be said for doing things right, not just what you can get away with or, in this case, without.