When someone recommends a service, the assumption is that they're linking because they really find the service useful. If it's a referral link, the issue becomes clouded - are they recommending it because they genuinely think the service is useful, or do they just want the cash?
I'm not against bloggers using referral links, but I think they should make it clear when they're using them. Their recommendation carries less weight if you know that they're getting paid for everyone who signs up through them.
Also, this line:
If you buy one of those items, you probably would have bought it anyway.
If this was true, no one would ever need to pay for advertising or referral links. But clearly, people do pay for advertising and referral links. Ergo...