Diablo – When should “the weary blacksmith” conversation in act I take place

While you might be able to miss a required conversation when playing strictly linearly, you don't have to play strictly linearly.

Diablo 3 is broken up into quests, and since you can start from any quest you've unlocked, you can jump back to whatever conversation you have missed if you go too far, too fast.

Once you've unlocked all the acts in a given difficulty, you can jump anywhere you want, at any time, so it becomes less an issue of "missing" a conversation than it is "figuring out which conversations I've yet to have". (And then jumping directly to the relevant quest)

You can make a very strong case that for the majority of users buying gear from the AH will put you further ahead as opposed to levelling up the blacksmith.

However, this relies on the prices in the auction house remaining low. This should be a safe, long term assumption as the supply should outpace the demand assuming people resell items they purchase.

The best gear that the blacksmith produces is certainly being hoarded by those who have invested the time and effort to generate it or will be offered for sale at an enormous price.

In the short term, yes, you can do better in the AH under the current economy.

In the long term, dependant very strongly on your luck in crafting equipment with the stats you are hoping for, the blacksmith may become your better source of equipment as the best Legendary items will almost certainly be absurdly priced.

Crafting equipment has always entailed some risk. In Diablo 2, the best players would hoard perfect rubies to try to create "Blood" gear (items with strong life leech). If your build calls for such a piece then it may be the small advantage you need to start succeeding in Inferno.

Many won't want to do this but will benefit from the few who are doing it. If they're looking for dex gloves and you want int, their failures are your opportunity to buy.