It makes sense that sexual reproduction is ubiquitous compared to asexuality. When selection acts on a population of clones, it's more likely that the population will be virtually decimated, increasing potential for extinction. Sexual reproduction, through genetic recombination, maximizes variability, which entails fewer individuals in a population being culled by a given selection pressure. Now, reflect on the risk involved with monoculture-agriculture; amber waves of identical phenotypes vulnerable to blights...I suppose we hedge our bets...
That said, I guess it could also be construed that the extinction risk of asexual reproduction is insurance against overpopulation, but it's arguably an overkill.
I normally don't get to much into genetics because I think genotype is only part of the story. Regardless of the cause, it's phenotype on which selection acts. The person who sees the approaching Hell Pig first is more likely to survive than her unfortunate peers, regardless of whether her superior vision is determined by genotype or the cultural suggestion that she wear corrective lenses (assuming, of course, that her lenses remain in place during her frantic egress).
(Tighten your croakies, folks--they want your legs on their pizza.)
On the other hand, artificial selection and genetic engineering, which gave us monoculture-agriculture, also provide the means to save us from the selection pressures that are worrying you. I think that many agriculturalists are worried about it as much as you and are being proactive about it. I wouldn't sweat too much.
Now, your hell pigs, on the other hand, look awesome. I'm looking forward to those. :-D
We have an advantage in how lethal parasitic lifeforms can become. If a disease is too efficient, it'll wipe out all available hosts in the area, before it can spread to another area. I'm reminded of a scene in the movie Outbreak, in which one of the doctors concludes that the disease much be man-made, because Mother Nature isn't this much of a bitch. That's a poetic way of saying it.
Are we working on a new strain of banananana that's immune to the blight?
The bit in Outbreak related to a man-made bio-weapon with pretty much a 100% effective kill rate, easy infection by respiration, and a very short incubation period ... something like a couple days. Rabies incubates for a few months. Rabies is also relatively difficult to catch, requiring direct contact with the infected animal, in the form of an injection, usually by bite. The biggest thing it has going for it is the ability to infect a huge number of species.
There are plenty of diseases with a 100% kill rate. Syphilis kills pretty much everyone infected (without treatment, of course) ... eventually.
Also, my post was mostly in response to the severity of the consequences that Tezcatlipoca was projecting. Sure, the banananana blight will wipe out a portion of the crop and cause price increases, but it won't wipe out our entire supply. A parasite that was that efficient would wipe itself out within smaller regions and die out, unless it could adapt itself to other plant species pretty damned quickly.
Extinction isn't a "strategy" really, its the failure of strategy. Therefore, it isn't that extinction of asexual populations is a way to control overpopulation, it is merely a weakness in asexual populations. A lot of really cool studies have been done with water fleas (Daphnia) that show how an organism with alternating sexual and asexual lifestyles adapt differently to the environment.
Asexuality is actually tried out relatively frequently by mean groups of organisms, generally scientists consider the reproductive advantage the main reason these groups tend to have early success. Nevertheless, some parasitical or environmental pressure tends to wipe them out as quickly as they rise up. So, in a way you could say evolution is constantly testing the value of sexuality, and so far it has won out.
Except perhaps in bdelloid rotifers.