Structured training without the issues (lag, can't quantify short hard efforts, drift over time, reading high for reasons unrelated to performance like heat) that go with heart rate.

You could also get one because it's cool and will give you an accurate Calorie count. I don't think that's any sillier than spending thousands of dollars on a new bike, over a thousand dollars on a wheel set, six hundred on a GPS bike computer, or up to a thousand to have enough attractive outfits for a week's worth of riding without doing laundry.

Provided it fits your budget there isn't anything wrong with that sort of thing - I run 2004-2006 Record components in shiny silver because the titanium hardware color coordinates with my frame, use 1997-2006 Record quick release skewers at $100 a set just because they look pretty, and tracked down a NOS example of my favorite 1990s jersey in a larger size which fit my belly.

A used Powertap for cost reasons. Could be just $200 in wired form with a little yellow computer. Functional but not sexy.

Otherwise good choices some where around $800 are:
- Powertap hub. One wheel, any crank, any pedals, swap to any 700C bike using rim brakes without tools.
- Power2Max spider based or Stages crank arm. One crank, any wheel, any pedals, swap with tools to other bikes
- Vector (one sided). Any crank, any wheel, one road pedal, swap to other bikes with tools.

If you switch between bikes with compact/standard/triple cranks you want a hub or pedals.
If you want to run different wheels for training and racing you want a crank or pedal based unit.
If you want to run SPDs for cross and other pedals for road you want a hub or crank meter.
If you want to switch cranks and wheels you want pedals (or the Brim Brothers cleat shim based setup which isn't shipping)

Power meters that measure power on both sides (crank spider, hub, two pedal set) will provide numbers more directly comparable with other power meters, and would be more accurate if your power balance changes.