As someone who runs I find the loose gravel surface more forgiving on the legs than tarmac. Another plus point for gravel is that you have no issues with an icy surface. This is a huge bonus in winter when many tarred surfaces are very dangerous.
I can understand why tarmacadam is used.
One thing about gravel surfaces is that they hold up better and are easier to patch up when the ground subsides and the surface breaks.
A good solution would be to have tarred surfaces around towns or villages and leave the gravel surface in the middle sections.
I think the "crushed bitumen is cheaper to maintain" argument was disproven in recent years though, the tarmacadam is apparently cheaper in the long run.
On the mixed surface solution you describe, Dungarvan has tar near the population centres and crushed bitumen/gravel thereafter. It doesn't negatively affect cyclists on 23mm tyres (the smaller road tyres used by racers): the only people I've seen with issues have been the elderly and those with mobility problems. They'll turn around where gravel meets tar!