I love my overdrive transmission, use it on the highway of course, but also in town if driving over 45 or so for any appreciable distance. If my ultimate goal was performance and reliability I would certainly put in a modern 5 speed, as they weigh less than an O/D unit and are probably a little more robustly built and reliable. But if my ultimate goal were performance and reliability I could sell my TR250, buy a Miata or whatever, and have change leftover.
Having the extra gear reduction is nice whether you get it through a 5 speed or an overdrive, the extra tall gearing is not needed in a TR as much as it is in a Spitfire or Midget, but still a welcome addition. Preference for the overdrive unit or a modern five speed is just that, preference. I always wanted a car with electric overdrive when I was young, still get a kick out of flicking the switch and feeling the revs drop.