But then you have to disapprove of a number of pretty damn nifty drinks. This is a problem...
Now, I am in agreeance with you when it comes to whisky, or more specifically, high-quality whiskies. They should not be mixed with anything but some water. I suppose the same can be said to go for a number of other high-quality spirits as well. However, the idea that one should not mix spirits with other beverages, alcoholic or non-alcoholic, must be discounted as just plain wrong and crazy talk.
First of all, a number of alcoholic beverages are already mixtures. Good examples are port wine (without with which we would not have port casks to keep our whiskies in, which, indeed, makes a number of the best whiskies actually a blend of whisky and trace amounts of other beverages (in sufficient amounts to give the whisky taste)) and Pineau des Charentes.
Secondly, there are a number of drinks and cocktails that are simply very good. One such is the martini, which admittedly needs not be a cocktail (the so-called Churchill Martini is in fact just gin. If the gin is high-quality, this is quite enjoyable, and also gives you a lot of weird looks). On the topic of gin, Gin & Tonic is a traditional and very refreshing drink on a hot summer day.
Now, a good cocktail or mixed drink is made not to mask the taste of alcoholic beverages, but to enhance the taste experience in some way. There are two different approaches here: Having the alcohol as the base and having the alcohol as a flavouring agent. A martini is a classical example of the former; the taste of the gin is subtly modified by the addition of the vermouth, making for a more interesting drink. A G&T is exactly the opposite; the gin is used as a flavouring agent to the tonic water, originally to mask the bitter taste of quinine, which was used as a medicine to prevent malaria.
In many cases of cocktails and drinks one can actually well defend the use of expensive and high-quality alcohols. This goes especially for the former kind, where the spirit will usually be at least 3/4th of volume (before ice, though most cocktails of this kind should be served up, that is strained; the drink will be chilled, but no ice is left to melt and dilute it nor keep it chilled to 0 degrees Celcius (I, myself, when enjoying a drink of this kind, will usually prefer it to heat up a bit from the shaking as warmer drinks taste more)).
This brings me to a sort of conclusion: The addition of other ingredients to an alcoholic beverage is most highly acceptable, when done for the right reasons. The right reasons should be to enhance the flavour and your enjoyment, not to mask the taste of the spirit so as to more easily get drunk. Using high-quality spirits can in many cases be defended; however in cases of drinks where the taste of the spirit is not the main flavour, it should usually be considered superfluous. That is not to say that you should use low-quality spirits; I do not condone the use of cheap and bad spirits for any reason. There is still a difference between making your Cuba Libre with a standard Havana Club AÃƒÂ±ejo Blanco and a AÃƒÂ±ejo 15 AÃƒÂ±os.