Your question is funny, I'm curious to know how did you came to wonder that /dev/urandom could be predicted.
But more than that, this raise an interesting flaw, as Wikipedia mention it :
In 2004, Landon Curt Noll, Simon Cooper, and Mel Pleasant tested a variety of random number generators, including the /dev/random implementations in FreeBSD 5.2.1, Linux 2.4.21-20, Solaris 8 patch 108528-18, and Mac OS X 10.3.5. They indicated that none of these /dev/random implementations were cryptographically secure because their outputs had uniformity flaws.
That said, I think it answers pretty well your question:
How much would that weaken a given encryption system?
I can't say how much, but I can say it would impact them significantly enough to not consider them as totally secure, because "none of these /dev/random implementations were cryptographically secure".