First a little background information:
A good battery will sit around 12.6 to 12.8 volts when fully charged.
When a good battery is put through a load test equal to its rated CCA (cold cranking amps) its voltage will drop to around 9.6 to 10.5 volts depending on the ambient temperature. It will then shoot back up to ~12.6 volts once the load is removed.
A battery with one or more dead cells loses around 2.1 volts with each cell that has died. A battery with 1 dead cell therefore has a voltage of around 10.5 volts, 2 dead cells = 9.4 volts, etc. But usually once one cell goes bad the battery is replaced before others die as well.
A bad battery can show a false voltage when it has surface charge, this occurs for a length of time after a battery has been charging. It can read a full voltage of 12.6 even though it has a bad cell.
However, when a battery with a bad cell is put under load, it will immediately fall well below its real voltage of 10.5 volts. Once the load is removed, it will only bounce back up to its maximum 10.5 volts.
So when is 10 volts enough?
To answer your question, 10 volts under a load test shows a good battery, especially when it immediately bounces back up to over 12 volts once the load is removed. 10 volts on a battery without load shows a dead cell and when put under load will usually fall well below 10 volts. A battery that shows a voltage of 12+ volts but falls below 10 volts under load and only rebounds to 10 volts after the load is removed is also a bad battery and likely has a dead cell.
Starting a car is very different from a load test. The battery is under load for much less time. A battery that falls below 10 volts on startup but that consistently starts the vehicle is probably either a little under charged or is aging and has lost some of its cranking power as all batteries do over time. Putting the battery on a charger will solve the under charged issue. If it is still falling below 10 volts after a good charge then the latter issue is the case.