Agreed, and they will also usually help in writing down the name of your destination, and making a note of the taxi. I seem to recall on more than one occasion seeing a driver dropping someone off being waved on by the hotel doorman instead of being allowed to pick up a new fare.

My one experience of knowingly using an unlicensed taxi driver in Bangkok was our own fault / stupidity completely. Leaving the Dusit Thani late one evening to return to our hotel (the Krungthep Wing at the Shangri La) we were approached by someone who said Taxi, we walked with him to his car (at the bottom of the drive (alarm bells should have been ringing) and realised when we got in that it wasn't a taxi but a normal saloon car. The sensible and logical thing to have done would be to have got out, but sometimes you don't do the right thing - maybe it was late, maybe we were worried about pi%%ing off the driver and now being out of site of the relative safety of the hotel entrance. Anyway, I think we were charged about 500 Baht to do the 2 miles or so back to our hotel (I guess about 3-4 x the going rate)