Modern belts are fine, as long as you keep up with the maintenance. Most 100s out there do not have enough miles to have needed this yet, or they're right on the cusp. But my question is, why should I have to do this?
These are the main reasons I have heard for the switch to belts by many manufacturers:
1.) Cost of manufaturing the engine for a belt drive system is cheaper. So are the belts themselves.
2.) Much quieter and smoother than a chain, hence their use in many luxury automobiles (ala Lexus and the origins of the 2UZ-FE).
3.) Lots of revenue for the dealerships (belt change is an expensive job - many service managers use the "scare" tactic to get people to replace them before they're due = $$$ for the stealership and a bigger bonus for the SM - actually, I'm just guessing on the bonus part...

).
A properly engineered chain, on the other hand, should last the life of the engine. Mine is still going strong at 150K miles. Also, chains will stretch before they break, so you have some warning as it start clattering. Belts can and do just snap with no warning. If you have an interference valve motor (which, unfortunately, the 2UZ-FE is), then you will have VERY bad engine damage if your belt snaps. Basically, the valves will meet the pistons, things will crunch, bend, and break, and you will be in for at least a four-figure repair bill.
Like I said, companies like Porsche, who have some of the best automotive engineers in the world, do not use belts, because they don't feel that they have to compromise quality and durability for a little extra profit or a quieter ride.