I came from a bike racing background, so was an early adopter of the power meter, dating back to something like 2003. Back then, I had a rear-hub based power meter from PowerTap — it actually still works, but lives on my road bike. When I truly got into this triathlon thing a few years ago, I bought a Power2Max crank-based powermeter. It, too, has worked flawlessly since installing it.
Obviously, you won’t be surprised to hear that I am a firm believer in the value of powermeters for the bike — both for indoor training and while out on the road, and definitely for racing. Once you’ve committed to power testing, you’ve got a really good baseline for power that works — regardless of wind, hills, or other drains on your energy levels. I don’t stare at the thing all the time, but it sure is useful at the start of a bike leg to help keep my level of effort in check… and then going through hills to help me not overdo them… and even on flats, where I get a good check on my tendency to really start to hammer.
As an added bonus, indoors I use trainerroad.com, which lets me tie a smart trainer and powermeter to the computer. This lets me use the powermeter on my bike for actual power measurements — and (kind of) lets me avoid having to calibrate the trainer all the time.
So, sure — you can certainly do without a powermeter in bike racing/tris/etc — but for the really relatively low cost these days, I can’t imagine doing so!