I, like you, had thought that I would never use a power meter nor would I ever race at my advanced age and fatness.
That changed when I started ridding with folks that would be doing training rides. I'd ride with them and they'd be doing exact plans of what they wanted to do to become faster. Hanging around them showed me the value of a real plan to get faster and that the investment might be worth it. I can say 6 months later that it did indeed translate into more endurance and speed.

HR will always lag your the effort that you put out and depending on the time lag you could be burnt out before your HR is at it's peak. A good explanation:

http://www.youtube.com/watch?v=cSqEAX9mRq8

So, in my opinion, if you want to get faster, you should invest in one and use it to train and ride. You will know your power zones and be able to ride within them.
For example; if I know my FTP is 200W I then know my power zones and can ride according to them. If I come to a long climb at the start or middle of a century I know I can put out a max of 150W, which is an endurance pace, and I won't suffer the rest of the ride.

There are a tonne of books and articles on training with a powermeter such as "Training and Racing with a Power Meter" by Hunter Allen and Andrew Coggan and a slew of stuff on the internet.

Good luck.