With web2py being licensed under LGPL it is possible to build applications which are proprietary software. To some degree it is even possible to build another web framework that uses unchanged web2py code as back-end. In practice, however, the latter is too complicated on a technical level. The most common case, direct modification to the framework, have to be covered by LGPL or GPL, so you cannot make a proprietary fork.
However, it is possible to create a a proprietary fork in the software as service model. So you can imagine web applications running on a fine tuned version of web2py while this tuning remains secret. Same thing may happen to Movuca if it is not licensed under AGPL. Somebody can take the code, make improvements and run it on a server without offering either binaries or code to her clients. Just the service. Free software movement sees that as unethical. You benefit from the community code, but your code is not shared back with community. With a rise of the cloud platforms this becomes more and more relevant problem and pose a risk to the free software. It is yet another way of circumventing the GPL and changing the free code into a proprietary one (there were others in the past that have been stopped by GPLv3 e.g. tivoization). As Mariano already pointed out, in case of web applications, the LGPL does have much sense as it is equivalent in this context to the GPL. And is always best to minimize confusion and use one of well known licenses rather than creating your own. So as long as you do not see proprietary forks behind server deployments as a problem, the best choice for Movuca is GPLv3.
