One problem that I can foresee using variables for this, is if you are running the code on multiple channels and/or multiple networks, especially if there is a large number of users per channel and/or a network that allows very long nicks.
Eg: 5 networks @ 10 channels/network, 100 nicks/channel & 50 characters/nick. Some quick math gives me a total of 255,000 characters, which, if all stored in the one variable, would exceed (or at least push) the current limits for the number of characters allowed in a variable.
