You hit the nail on the head there, because ultimately this is a question of cost. The fact of the matter is that if an SRM cost $50 like a heart rate monitor, "should I get a powermeter" wouldn't even be a question. Everyone who uses a HR monitor would have one because it IS a very useful piece of equipment. It is just not worth the cost to some people. Is it absolutely essential to be a good or successful rider? At the amateur level, no. Will it help make you a better cyclist faster if you use it for training? Yes.
In in terms of why it is useful to use with a power meter, many of the authors of these various books describe it in a way I don't totally agree with but it gets a point across that the data is different. They describe the HR monitor as measuring the "input" or the effect that a certain workload is having on the body, and the power meter as describing the "output" or the work that your body is actually doing. I suppose this is one way of thinking about it, but in my opinion, I also think of the HR monitor as measuring the effect that your leg work is having on the heart's response to the increased need for oxygen, but in a "delayed reaction" fashion because it takes a while for the heart to respond and recover. They are both very useful to have, and I would not say one is more useful than the other. The point is that they definitely measure different things, and when used together also with RPE, are the best training tools we can use to improve our cycling (along with GOOD analysis software like WKO (NOT strata premium), someone who is expert at interpreting that data and coming up with plans to correct deficiencies... A GOOD coach who knows what the hell they are doing, which is much more expensive than a powermeter, by the way, and you're NOT going to be able to pay for his services by simply patching all your tubes, OK, durianrider?!).