Why would you get banned for logging on with a usable nickname?
A number of channel ban on nicks with numbers for a few reasons.
1.) To keep underage people out. Any nick ending with a number less than 18 (or sometimes 21) gets booted. (Not a great policy, but a number of channels DO use it.)
2.) The girly drones all have numbers attached to nicks. While it is very very VERY easy to make a proper regex and ban only drones this way, not everyone knows regex (or how to script properly). Instead they ban anyone with a nick ending in a number from 10 to 29.
3.) A few channels use it to prevent known flood patterns. While not so common today, there was a point in time that it was fairly common to see a bunch of drones with the same nick, but different numbers attached to the end. ie Drone65, Drone728, Drone 092 etc. A number of channels ban on nicks ending with numbers to deal with drones.
Now none of the three policies above are very "good" as they can be modified with scripts to cut false positives down to 0 (or very very close to it at any rate). However, since most people who use mirc don't know much about scripting, we're left with some poor choices to deal with the above problems.
So, that in a nutshell, is why people get banned for having numbers attached to their nick.