Although I have nothing against the train it does have its own annoyances and disadvantages:

• I was early for the first train to AMS at St. Pancras, but security and passport control where very crowded and cramped. Although pretty quick not very comfortable.
• There was no seating in the waiting area available whatsoever, every seat was taken.
• I found the seating (at least seated vis-à-vis) uncomfortable
• Although there are cheap tickets, last minute is usually expensive and business seating even more
• The internet in the train is useless (and mobile reception in general is quite bad), you could argue it's the same on the plane but then its < 1 hour. I need internet to work, so for the same travel time I can use an hour or more in the airport lounge being productive (if i choose to be that early at the airport).
• Also, at Schiphol I can use Privium for security and passport control

YMMV, but blanket statements are rarely true for everyone