Pre-mixing liquor -- any downsides? I love Long Islands.  In fact, all I make are Long Islands.  Yet every time I make one, I have to independently mix the four liquors I use:  Vodka, Tequila, Gin, and Rum.  (plus I add lemon juice and splenda)   

It seems to me it would be easier for me if I could pour all four bottles into one container and just pour my drinks out of that.  (and store this container in the fridge indefinitely until ready for use)  That way I don't have to worry about proportion (since I always mix them equally anyway).   Is there any downside to doing this (aside from the obvious downside that I can no longer pour any other mixture)?
There should not be any problems with mixing alcohol as long as you don't introduce organics.  I would suggest mixing the alcohols and leaving the lemon juice and splenda out until you pour a drink.  Keeping alcohol cool and in a dark place.  As for indefinitely, no.  The amount of time it stays good is dependent on the amount of air in the bottle. Glass is always preferred if possible.