You're right - phpBB doesn't (in the default install) support forum-wide subscriptions (unlike most other forum software, like vBulletin or Snitz). However, forum-wide subscriptions are pretty common (I use them to get emailed posts on about 10 forums I watch, which results in hundreds of emails daily)... yes, it increases the email load going out from the forum server, but it's a great feature for someone who wants to read every post on the forum, but not have to remember to come here and figure out what's new.
Looking on phpBB's list of addons, there are quite a few that support forum-wide subscriptions... so, if you ever do any modding to phpBB, it'd be really nice if it included adding forum-wide subscriptions