Here are some reasons, in random order:
1. The courses are "immersive" with frequent short quizzes, explanation of answers, etc. (in case of udacity, it is almost like once every couple of minutes). This is a big plus compared to correspondence courses.
2. There is a strong online community, instant access to reference material, forums, discussions, etc., which is a big plus.
3. Most of the material is free (I do not have any experience with non-free material).
4. The teachers are top class -- I mean, really top class, and the material they teach is high class and very unique [*].
5. The classes are massively scalable, archivable, easily made available, etc. (correspondence courses aren't).
6. There is an Indian saying "knowledge is wealth". So far, the top 1% have rarely helped the bottom 99% (and made them think that they should only "occupy wall street"). The MOOCs help in making the knowledge available to the 99% (turns out, it is a simpler problem to solve than the financial one).
The only major point people make is with respect to evaluating the credentials of a student who has taken these courses (and any types of cheating)... It is not a problem of the educator -- my belief is that the job of evaluating a candidate is mostly that of the interviewer. Employers that rely on lazy interviews in hiring people help the society at large -- they take away people that game the system out of the pool! And, slashdot should be the last place where education becomes secondary to grades (mind you, there are still grades for the MOOCs, and one can repeat the courses multiple times -- so one actually learns and deserves a top grade).
[*] To give a perspective, I am old, not from comp.sci background, didn't know python as of January (and have been destined to amount to nothing much!). I completed two courses on Udacity (CS101 -- thinking they'd focus on search, but they taught me python; and Peter Norvig's course). I had a phone interview with a "big deal" company where I gave a one-line answer based on what Peter Norvig taught [which impressed the interviewer -- and I explained him that their guy taught me the stuff]. I also took a course with Tim Roughgarden on Algorithms, and that helped me re-discover the joy of math and formal treatment of problems. I met him [Roughgarden] recently when he was visiting a nearby university, and his point was, if someone spends one hour on his class and learns something, he is more than happy. Without these courses, I'd still be wondering, "where did I screw up". Not any more.