Mmm, not in all cases, Gef. The quantity of liquid can be important. It's not enough just to make it easier to swallow - some drugs can cause big problems if they're slow getting through your oesophagus, so it's vital that they are washed down into the stomach as soon as possible. For example, one drug that I know of causes ulceration, and the instructions specify that you must drink an entire glass of water, then remain upright for some time afterwards. Now, drinking an entire glass of something fizzy would make me feel rather queasy, the tea would have to be cold to avoid scalding the innards and frankly I don't think I could drink that much juice at once!
Also, what's in some drinks can interfere with the absorption of certain drugs, eg milk protein binds to some of them and means that the drug isn't absorbed properly. Caffeine can reduce iron absorption, although Vitamin C can help, so they've told me to give my toddler her iron syrup at the same time as fruit or fruit juice.
Summary: being relatively inert, water is about the safest thing to take most drugs with, and is unlikely to affect their effectiveness.