I have no problems using public transport in places where it is readily available and easy to use, and my criteria for choosing between public transport and taxis are generally similar to yours.

When flying into out-of-town airports with mainline rail links (such as AMS or FRA) I will always take the train into town even when carrying a suitcase - but may well decide to then use a taxi to get to my hotel. If getting into town involves a bus ride, or several changes of transport, I am more likely to take the easy way out and go by taxi... though, having said that, I recently used public transport to get to Hamburg Airport from the outskirts of town, a journey involving two trains and a bus!

Visiting clients in New York a couple of years ago they seemed most surprised that I had taken the Subway for the short journey to their office from my hotel... but they seemed to take my point when I explained that it would have taken me twice as long to get there by taxi!