You've answered your own question pretty much.
The closed deck inserts weren't available in the UK when I built this. It's only recently they've started to do them. The inserts provide the strength where it's needed without restricting coolant flow.

They were used by few well known tuners in the UK to good effect in their 800+Bhp race cars. The dowels were easier to do, cheaper and did the same job. The block I built is 6 years old now and is still going strong at just under 600bhp. I would probably do another but I really want to do the 2.1. As didnt get to finish the one in the first set of pics due to moving here.