If my experience with CMS and other goverment (F, S & L) is any guide, you'd have to have extensive documentation about their particular applications running on everything from Oracle to DB2 to VSAM tables, taking input from ONC RPC calls, ZMODEM file uploads, CORBA requests, and TTY emulators. Not exactly something that you can just jump in and work on.
Beyond that, even if h/c.gov was, say, thrown up on GitHub, you couldn't just tweak some files and issue a pull request; everything has to go through FISMA audits, which make your traditional waterfall change request look like updating a blog post.
But the code for the marketplace isn't.
I think if the marketplace was open bug fixing would be faster.
1. "Scratching your own itch". There are several hackers / freelancers that want to enroll in healthcare and want that website to work.
2. "Altruism". There are plenty of hackers that want better healthcare for the USA and would gladly help to make that happen. Even if they aren't from the USA.
There are already so many hackers working on open source projects for the government, even healthcare in government, that there would be no shortage of willing helping hands.
[1]http://reporting.sunlightfoundation.com/2013/aca-contractors...