The current installation process does
invite the user to subscribe to a filter, but in my experience with the users of that site - who are mostly newbies and really don't understand computers at all - they often fail to do that step. I don't really know why - you'd have to observe people doing it to find out - but I suspect that there are three reasons:
1 They get the "1 new add-on has been installed" message and think that's all they have to do.
2 They don't recognise the subscription webpage as part of the installation, or relevant - if they notice it at all! And if they read it, they don't understand it.
3 Many of them are fearful of installing things - and especially 'subscribing' - for fear of malware, so if in doubt they don't (which is not a bad principle!)
are the screenshots I did for users of that forum to help them, so you can remind yourself of how the install process works.
is another typical thread where someone failed to subscribe to a filter.
For many users of that forum - and some other forums I use - ABP is the
reason they're installing Firefox in the first place. ABP is the first add-on they've ever installed; they just want to block the ads on the forum (usually prompted by a particularly irritating one). They're not only unfamiliar with the add-on installation process, they're unfamiliar with Firefox. And if ABP doesn't work first time, they've no reason to persevere: they give up and go back to IE.
Personally I think that subscribing to a filter by default is the way to go - experts can surely opt out. And obviously the localisation is important too.
Sorry to go on at such length, but I think this is important for Firefox - not just ABP.