I think his conclusion about the probability of intelligent life is a bit pessimistic.  After all, at best, we can only hope to ever explore our own galaxy incompletely.  There are so many galaxies!

However, on a functional level, I think he's right.  We will almost certainly never encounter another intelligence.  For all intents and purposes, we are alone, and environmental or nuclear cataclysm is the worst of all possible fates.

Dude.  This is a good site.

I think you are correct, from a biological standpoint, the proposition of only being a singular intelligent life form is dubious at best. In the fight between the Anthropic Principle and the Principle of Mediocrity, the more intricately complex something is, the less likely it is to be the result of one-off events. This is tidy logic, because opposing it will commit the gambler's fallacy. It is exceedingly unlikely that biological life arose from a series of random fluctuations in entropy, for example. Such existences necessitate non-random processes and guidances for their existences. Similarily, the idea that Dawkins presented, that perhaps life is ubiquitous but conscious life is unique, is on very shaky ground for the simple reason I just described above. The mere fact that there are so many opportunities for it to arise does in no way provide a satisfactory explanation for why it is so complex, in much the same way that whilst entropic fluctuations could describe, say, the random coalescence of water into small pockets of ordered ice within a glass could be explained, but certainly not the whole glass freezing if we wait around long enough (the mere addition of more time or glasses by itself would not increase the probability in any way). 

I thought Dawkins was very wrong in the God Delusion the way he described the Origin of life as an exceedingly improbable event that kick-started natural selection. Chemical evolution is a well-established principle, following precisely the same laws of natural selection as does biological evolution. It is not a game of accepting one or the other. The logic is the same, and you must bite the bullet if you ditch chemical evolution.  

Two words: No data.

Carl Sagan responded in this way to a reporter who asked him the same question, that there was no evidence and no data that could help produce any number or answer as wanted that would not be wild guesswork. And the reporter pressed Sagan for his "gut feeling" on the matter.

Carl responded with something that has since become laconic legend: "But I try not to think with my gut".

My answer is the same.