Daniel, sorry maybe your question got lost in the For/Against camp debate there.
For what it's worth, I've found training with an HRM really useful. When I got one I was running regularly, but without structure, and the hrm told me I was running practically all my runs at the same effort - the effort normally reserved for so called recovery days, or long slow runs.
It was absolutely brilliant for mapping out the actual efforts spend during a run. I live in a hilly area, have no access to track etc so I run lots of different routes and my pace reflects that.
During my first marathon training I used it to work out what my target marathon pace was as a % of my hr, and I used it to pace myself in my first marathon to good effect.
I'm sure some people can achieve the same with only their stop watch, but it's certainly been well worthwhile for me personally. I would say you need to buy your own, establish your individual training zones, try it out for a few weeks on different runs to gather some 'base data'. Just using it once or twice is unlikely to tell you much of use.
It's been said before but I'll say it again, it's imperative you work out what your personal max heart rate is and don't rely on the formulae which are notoriously inaccurate judging by the number of queries about this!
You could get Parker's book that sfh legs recommended, and see whether you're convinced it would be a worthwhile buy or not.