a) Adblock Plus 1.3.5 will be out (significantly?) before Firefox 4.
b) Firefox 3.5/3.6 users as well as SeaMonkey/Songbird users might be interested in protecting their privacy as well.
c) Anybody who uses EasyPrivacy already indicated that he cares about his privacy, why require them to check another option?
d) The Adblock Plus feature is way more flexible than the planned Firefox feature and allows exceptions on particular domains for example (whether this will be necessary is something we still need to see).