Certainly Iran has 'rational' reasons for seeking a nuclear weapon, that doesn't mean they should have them. I think an absolutely massive facet of this many overlook is the way in which nuclear weapons would shift the strategic calculus in the region, particularly from Iran's perspective. My point, and the point that many have made, is that a nuclear Iran obviates Iranian fears of regime change and significantly reduces their concern over a conventional strike on their territory.
As a result of this new-found security Iran would be able to more aggressively pursue its regional objectives, which revolve around regional hegemony. It's ability to project proxy forces would aggressively increase on critical fronts including Iraq, Afghanistan, Syria, Yemen, Lebanon, and the Palestinian territories. Furthermore its ability to use its nuclear capability as a buffer would significantly increase their propensity for launching limited conventional attacks at Gulf targets, or along its periphery in reaction to geopolitical tensions. The underground battle for Iraq being waged by Iranian and Gulf (primarily Saudi) agents, donors, and proxies could become overt. So many things could happen, and we can pretty clearly see many of the ones that would happen, it would be a terrible thing to allow to occur.