Some friends that know me well might tell you about one of my "termite killing frenzies" with me chasing after them when they come out to breed. There are plenty of them around here, and those of us with any sense know that there always will be.
Legend in California is that redwood is termite proof or some such thing. I've seen them eat redwood, so I know that's not quite correct. They might not go after old growth heartwood, but the stuff that's available now is a different story. The thing is, most woods once they get some amount of rot going will be a target for insects or beetles.
The basic thing with fence posts is to use a wood that isn't prone to rot and weathers well, like cedar. Using several inches of gravel under the post as well as around it helps drainage a lot and keeps the post from rotting. A concrete cap locks it in place, protects it at ground level and directs water away. Not anything terribly complicated, but I see a lot of contractors mess this up and actually cause problems.
Then you have the people down the street that just replaced their fence which rotted away prematurely because their sprinkler system gives it a good soak every morning. A daily soaking/drying cycle is definitely not good for wood. The contractor that installed the new fence did a horrible job on setting the posts and it will follow the route of the first fence in no time at all.
I agree that you probably won't find me using the sawblade I've reserved for joints or veneer on wood that might have dirt embedded in it, but I probably wouldn't buy a special blade for it either. In fact it would never occur to me, possibly because there aren't cheap blades that will fit my saws. I think my average blade price is about $150, but they don't sell those kinds of blades at home depot either, so my options are more limited than yours.
So perhaps the reason for your recycling/reuse dilemma is really just that you didn't spend enough to be free of it in the first place!
Seriously though, when faced with the question of buying something really really cheap that has other alternatives, I always have to ask myself "who made this and how were they paid for their work?" If the answer is that they weren't paid enough to live at what I consider a minimum standard, then I don't buy the product. I don't like having to consider these kinds of questions when I'm shopping, but I do ask them.