I wasn't counting them and most people don't. Microsoft does because they argue it is only for HD DVD playback and it does have a better attachment rate than the PS3 when it is used as a BD player.
I agree with you that since the add-on is specifically purchased with movie-playback in mind, that it accurately points to a meaningful attach-rate figure. We can only hypothosize about attach-rates for BD given the uncertainty about what percentage of PS3 players are used for movie-playback (corresponding with add-on use) and those used primarily for gaming. However, one must also concede that leaving all PS3s out of an attach-rate evaluation, while applying 360-add-ons for the HD DVD camp, skews the comparison between the two since it omits a meaningful portion of BD movie watchers, even though that exact figure cannot be determined.