diz, to answer your question, sometimes this situation occurs:
You don't want population b/c of happiness
You don't want workers/units b/c of gold/demographics/etc
You haven't invented Research or Wealth yet
Thus, the opportunity cost of pre-building stuff is zero, when compared to producing nothing, and it is only advantageous.
You are correct, in that it if you need a worker, it is better to:
1. Build a worker
2. Build a unit
as opposed to
1. Pre-build a unit
2. Build a worker
3. Build a unit
However, the first situation does actually come up in games, especially for production-heavy cities, or on "quick" settings.
You don't want population b/c of happiness
You don't want workers/units b/c of gold/demographics/etc
You haven't invented Research or Wealth yet
Thus, the opportunity cost of pre-building stuff is zero, when compared to producing nothing, and it is only advantageous.
You are correct, in that it if you need a worker, it is better to:
1. Build a worker
2. Build a unit
as opposed to
1. Pre-build a unit
2. Build a worker
3. Build a unit
However, the first situation does actually come up in games, especially for production-heavy cities, or on "quick" settings.