Rebates are, plain and simple, incentive to get you to buy Product B over Product A. Take for example two 100Gig HDD's, next to each other on the shelf at Best Buy. One is $120. The other is $125 + a $40 rebate. In essence, the manufacturer of Product B is willing to *pay* you $40 to but their product. Sure it'll cost'em (once you prove to them you did buy it), but it goes towards their bottom line.
You're right about technology and the fact that it should be speeding up the rebate process. Well, it does.. in terms of facilitating the job of the $10/hr employee behind the computer screen. And I think this is where RebatesHQ is the leader. However, consider the fact that often times there is backlog. More rebates submissions than are able to be processed in a timely manner. Technology can't do much in this case. More manpower can.
I think in the past that most rebate processing companies intentionally delayed payment so that they can earn money off of interest. However, rapid technology does not behoove this. I think in RebatesHQ.com's case, 99.9% of the delayed payments on otherwise valid rebates is due in large part to participating companies (OfficeMax, Staples, etc) not funding the accounts that your payments are cut from in a timely manner. Remember, RebatesHQ only pays you the money provided to them by the manufacturer. And the fact that it shows up as a postcard instead of a first-class item is because of the large difference in postage. Remember, not only are you trying to save money, but so is RebatesHQ. With RebatesHQ (and I'm not sure who else), I don't think it's in their best interest to hold on to or deny your rebate on purpose, since I think they make their money based on turnaround (ie, the number of valid transactions).
Just remember to hold on to all your paperwork, since it doesn't take a rocket scientist to sit behind a computer screen and enter your information.
my $0.02 (after rebate of course

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FWIW, I submitted a Best Buy rebate via RebatesHQ.com in late January and received my check 4 weeks later.