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The drafting effect you see along the side of a semi-trailer is noticeable as you mention but it is not as strong as directly behind the trailer.
Aside from the fact that to effectively side-draft a truck you'll be riding in its sizable blind spot I do think that this is probably safer for you as the drafter, but will probably tick off the truck driver more that you're impinging on his freedom to change lanes.
Point of interest: in my college Vehicle Dynamics course we had a class project to perform an experiment and test an hypothesis. A group that one of my friends was on went out on I-81 and drafted semi trucks at varying distances behind the trailer while recording instantaneous MPG readings from the vehicle's OEM display. They recorded copious data and determined that even at following distances of up to 2 seconds at 65 mph there was measurable MPG benefit. Slightly more than 2 seconds produced an MPG *decrease* and somewhere around 2.5-3 seconds MPG was the same as without drafting.
So keep in mind all the knee-jerk "never draft you reckless fool" types; ever slightly closer than the standard 2 second following distance will improve your MPG via drafting without endangering anybody. Try counting out a full two seconds on the highway and you'll see the following distance invites people to enter the space.
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