I'm not sure if there's a cause-effect relationship here. MakerBot was only founded in 2009 , RepRap released their first project in 2007  and the project I was a part of, Fab@Home, open-sourced in 2006 .
I can't speak to the other projects' motivations, but I know that the fact that patents were expiring made little to no difference on our decision to make a printer. If you can believe it, the Fab@Home was almost an afterthought. We had top-of-the-line commercial 3D printers in the lab, but they didn't do the kind of custom material jobs required for the project that Evan Malone was doing. His PhD project was to create a 100% printed-from-scratch robot, and no technology fit the bill. So, he and Hod built one in order to get that project done. Like the Oculus, technology had advanced to the point that doing this was actually feasible on a shoestring budget. It ended up being a project of its own, but it wasn't like they saw the patents expiring and all of a sudden decided it'd be great to open-source a 3d printer design.
With how popular 3D printers are getting nowadays, it's possible some companies are looking to take advantage of the expiration of SLS patents, but I wouldn't hold your breath for an open-source project. If someone was making one, you'd know about it already.
I was involved with the RepRap community in 2008-2009. The expiring Stratasys patents for FDM (fused-deposition modeling, the squirt-out-hot-plastic approach used by the RepRap, MakerBot and its clones) were definitely a big deal in the community at that time. Adrian Bower, the creator of RepRap, didn't seem to care about them much but that was because he was never interested in making money off of the project anyway, just releasing its plans to the world. It would be hard to sue him for infringement. But other more commercially-minded people were definitely drawn to the opportunity afforded by the expiring patents.
On the other hand, I agree that only relatively recently has this become feasible to do on a shoestring budget. So there are a number of factors involved.
Giving plans away for free would not make it any more difficult to sue somebody for infringement. More likely, the patent holders simply didn't see it as a threat to their business (yet) and let it slide.
Would think so, but IANAL and I'm not sure about the legality of it. Infringement occurs whenever you make, sell or use the invention without license. Distributing plans seems like it would contribute to infringement, so some enterprising lawyer could make a case for it. It's like linking to infringing copyrighted content.
I recall a story about a guy who blogged about coding up a shazaam clone and then receiving a C&D from the patent owner. It was never settled whether he had any grounds to do so, but sending C&Ds is cheap and mostly consequence-free.
Patents do not restrict rescues for research purposes, true. That is the one widely acknowledged exception to infringing use. That is why you can study an invention and then patent an improvement or novel enough variations thereof.
But distributing detailed plans (or source code, as mentioned in sibling comment) of something that enables others to easily recreate something covered by a patent just seems to be less clear-cut to me (IANAL).
As a member of one of the biggest groups of Rep Rap members(in the thousands) I find patents really important.
Patents forbid you selling anything, even with no profit.
Lots of pieces need to be sold for making 3D printers accessible for all. The electronics controlling board, the hotends, the friction wheels.
Most of the general population can't fabricate those things, but they want to use 3d printing anyway, like most women(90%of reprap now is men) don't know(or care) what a fuel injector is but they want a car for going to work.
In the future it is about Inkjet cartridges, wax and ceramic binders.