This "excuse" for an argument is just not right. Consider this:
There is a difference between justifiable and just. You have found justification for your actions and are somewhat arguing that that alone makes them just. Yet, your actions simply are not just.
It is never a good thing to defend illegal actions by comparing them to illegal actions followed out by others. There are several differences between your illegal downloading and others' activities. If one were to jay walk, they are either not-knowingly doing it, or they are ignoring the law but are acknowledging that what they are doing is mostly only putting themselves at risk. By downloading, you are theoretically stealing the work of others for your own benefit and/or distribution.
I hate the RIAA with a passion, but their targets are equally lame.