Do I -NEED- a heartrate monitor?

No. 

Most humans have done well enough without one for the last couple thousand years, including generations of World, Olympic, Kona, and age group champions. Don't know why one would be needed now. 

What is making you consider buying one? It sounds like you have a motivation already and you want to pull the trigger. If the motivation is that you read about a great HR zone training plan and you think its going to improve your athletic potential, then I say, by all means make the purchase. If the question is whether or not to purchase a gadget to help you use a methodology that may improve your fitness and provide you with additional incentive, then I give it a green light. If you want to get one because all the cool kids have one or b/c of the novelty of it, I'd say don't bother.

I'll try to quote a post I saw last year sometime: It is imperative you use a GPS and HRM, it is quite possible you would have a heart attack and run off the edge of the Earth at the same time!

Of course it isnt necessary, it is interesting and if you use it correctly helps structure your training. I quit messing with it for running, I use pace or rpe. For cycling i like it because it keeps me honest and I have made a commitment to make my limited saddle time more beneficial.

wrong question.

the question is:  Would you benefit from having one?

I think that enought studies have shown that training by RPE is less accurate.  More often than not, my HR monitor is what tells me to slow the f(&^ down for my training runs that are supposed to be slower.  Yeah - you could do it by pace... if your route is flat enough.  Mine isn't.

Agreed. My RPE is only ok, but it's much better than it was before using the HR monitor. Horrible when I started out. My sprint power and level of excitement made it go batty at times. Very little consistency. Others don't have this problem and can do quite well without it.

I can't remember the formula but it some some math equation off resting HR to figure HR zones.  I found that the monitor agreed w/ my personal RPE formula:

Can sing, hold a conversation (feel like I can hold this pace for a week) = Zone 1 to 2 +/-

Can speak in single sentences intermittently (feel like I can hold this pace for half an hour) = Zone 3 +/-

Can't speak, can only concentrate on breathing (Feel like I wanna puke) = Zone 4 +/-

This formula may not be as exact or satisfying to some as the whiz-bang electronic gadgets but it works for me and it allows me to space out/meditate more and look at the whiz-bang electronic gadgets less.

As I tell mrs gearboy, "need" is a subjective term. So really the question should be "what niche will the HRM fill for you?" Granted, this kind of question is why I own 3 kayaks, 7 tents, 5 stoves, 3 sleeping bags...

I like having the data my Garmin gives me. I can instantly see how far I am going, what my pace is like, where I am in my HR (usually too high). I can download it to the computer and compare my progress (or regression). Some people like to run for fun, and do not require a lot of motivators. I am not one of them.

i have 2 and they never get used

(inb4 "can i have one")