If someone was directly targeting me, and had my email address from another site, they could probably figure out what I'd used elsewhere. But if it's just a script running through email addresses harvested from site A, then mine will almost always be irrelevant on site B.
The main reason I use it though is that it's a great way to figure out where spam is coming from. Last week for instance I got a "male enhancement" spam from an email address I've only ever given to scan.co.uk. That addres is now on my block list and I doubt I'll be buying through them again.
Sadly this is at best a complicated workaround, that will will work for people that are motivated enough to remember for each different service a separate email and password and additionally to this you have to remember as well the credentials to manage your email address and check the emails from different sites.
In my case it would mean having about 100 email addresses.