The effectiveness also depends what they're trying to gather. If they're just looking for statistical data (e.g. to sell, which is something I suspect ISPs are

The effectiveness also depends what they're trying to gather. If they're just looking for statistical data (e.g. to sell, which is something I suspect ISPs are very interested in), the random browsing can be made to cancel out in a large enough dataset. If it's your government focusing on you in particular, they're probably more interested in forensics/malware on your clients so they can see HTTPS stuff too.