Depends on your build strategy. My spreadsheet says don't leave home without at least one.
Yeah I don't see how it works out. In a 10-game campaign, the earliest you can buy it is after rd 2. If you do, you get 7 turns where it generates money for a net of +2 gold. That's a terrible return on investment in the best case scenario.
In our 6-game campaign it made slightly more sense. We doubled income earned every turn and income earned from the smelter/saw mill. In this case, if you buy after rd 1, you have 4 turns of earning 2 gold a turn for a net of +3 gold by campaign end. That's just barely acceptable, and only if you buy it after turn 1.