The element of time...This model seems to make a lot of sense. One of the traditional weaknesses of the LMS (layer one) from a learner's perpective is that it is so focused on a course, and you'd lose writing (AND READING!) access to the site after the course ended. I think one of the greatest benefits of integrating Elgg in layer two is that learners can keep their learning active throughout a program, or even years after a specific program has ended. In terms of keeping access to specific materials, showing progress in learning over time, and developing a learning network, this is the crucial layer.