One thing people tend to forget about "alternate biochemistries" is that while they might be possible, they tend to be quite chemically limiting. For example, water surpasses methane as a solvent, and its liquid range is at a far higher temperature, allowing faster/higher chemical interaction. Methane is also liquid over a short temperature range, which makes an ecological reliance on it pretty tricky.
Silicon also has problems. For example, it is not as chemically active as carbon, is less abundant, and its oxide (silicon dioxide, or sand) is not soluble in water.
It is for reasons like these, among others, that make it improbable for 'alternate' life to evolve into complex, advanced organisms that might build spacecraft, for example. It is not narrow-mindedness, rather, they are at a simple and clear chemical disadvantage to life similar to us. I'm not excluding anything that is not carbon-and-water-based, but rather excluding most, if not all, of the currently proposed biochemistry concepts.
Life existing on any of the noble gases is, dare I say it, impossible. These are gases that are inert, they will not even interact with the environment or form any sort of biologically useful chemicals, let alone be metabolically advantageous.
It is also not egotistical to suggest that life chemically similar to our own will predominate. Hydrogen is the most common element in the universe. In our own galaxy, it is followed- after inert Helium, by oxygen (third most common element). Carbon is the fourth most common element. Nitrogen is the seventh.
All the components of CHON are in the top ten most abundant elements, and all but one of the components of CHONPS are in the top ten- sulfur makes an apperance as #10, but phosphorous is nowhere to be seen (though presumably is not that far down the line). This is not critical as phosphorous is not required that much by living things, and even then, alternatives like arsenic are probably far less common (phosphorous makes up some 1000 ppm of the Earth's crust, arsenic is only 1.5-2.5 ppm).
That doesn't mean that extraterrestrial life could not be chemically novel- they could be made out of the same (or similar) chemical "building blocks", but arrange them in totally different ways. The side effect is that alien sirloin steaks could be fulling without having any calories (alternate chirality of the molecules), or could even kill you, act as an illicit drug, or be medicinally important (have natively common but otherwise unknown biochemicals with unforseen effects).
