As you noted, crafting can be maxed at any time. Block isn't particularly difficult to level; just grab a shield and go let a giant beat on you. Light/ Heavy Armor skill does take some time to level, but we don't need to have everything at 100 by level 15, do we?
Using a shield certainly makes you more durable in the early game, at the expense of significant chunk of damage output. The problem is that once your crafted armor is mitigation capped, and your Block is maxed, your shield's main benefits disappear; all that extra armor value is wasted, and the higher base Block % is irrelevant because your Two Hander blocks just as well. And how were you spending those perks in the early game? Filling out Block, One-Handed, and Two-Handed won't leave you much room for anything else.
Since we're trying to optimize here, mid- and late-game are definitely out targets, but you're not really gimping yourself in the early game by using a Two-Hander instead of a Shield.
The extra enchanting slot(s) are nice, but they're hardly a compelling reason to use a shield. +Block enchants are usually wasted on a Shield; a lowly Hide Shield starts off with a base Block % of 50, and 1/5 Shield wall brings that up to 70%-- just 15% shy of cap already. Upgrade your Shield once or twice, and you're already at the cap without any enchants. An extra slot for Resist Magic isn't bad either, but between the Lord Stone, Agent of Mara buff, and Alteration perks, it's not really necessary.
Good stuff on this thread! Still I am playing mostly-organic ( a little training block and heavy armor here and there) and just hitting heavy armor + block at 70 at level 30. I think you are down playing how long it will take to hit the armor/block cap. Unless you just flat out straight craft and let a mud crab hit you for an hour. Which is not-fun. My personal opinion. I am curious what level people are hitting when they max out the three crafting skills? And if you are on the PC I am sure most people here will use a mod to nerf some of this stuff. It is only fun to be god like for a while. Ha, I am sure someone will use this thread to pick out exactly what needs to be nerfed! (Mostly just eliminate/cap + foritfy skill enchants/potions, cut smithing dmg/armor bonus in half- poof a much better game)
Plus a shield gives you acces to two extremely powerful perks - Elemental Block and Shield Charge. After doing the basic math crunching here for a super munchin character it would be interesting for the people on this thread to post what are the "best" perks in the game. Besides enchanting, smithing and alchemy!
On the subject of bashing - every test I have done shows two hand weapons doing a lot more damage then a shield. Daedric War Hammers hit for about a 100 and Daedric Shields hit for 40. That does sound a little - wimpy compared to the super min-maxed numbers posted here. But good damage.