I've played many, many games of TM in real life, often against an opponent who is better at strategy games than I am. We find that most of the time we play two actions each turn, but there are some important exceptions:
- When you are trying to position yourself to get a bonus (i.e., the ocean bonus for raising the temp to 0C)
- When you have a card with a requirement that hasn't been met yet, but you could play if one of the other players fulfills that requirement on their turns before you're forced to pass
For the second point, this only works (for us, at least) if we can "stall" a little bit but not too much. If we realize the other person is stalling and can figure out why, we're mean enough to choose not to help them!
We also will take one action if we are trying to figure out what moves we want to make and it's taking us a long time. In real life, that actually keeps the game moving, letting the other(s) play instead of waiting for a very analytical player to figure everything out in their head. This really shouldn't ever apply for online turn based games. I could see in an online real-time game someone running out of time and choosing to do just one action rather than take the time penalty...
TL;DR: my opinion is that most of the time, it makes sense to take two actions, but there are cases when 1 is a useful strategy. We don't have any patience for players selling cards one at a time though!