I see two major drawbacks that result from their decision not to switch to GPLv3:
* It is a loss for everyone in the community. The Linux kernel won't be protected by the additional clauses in the new version of the license.
This implies that Linux can be relicensed, it cannot unless you get written permissions from anyone who has contributed a legally significant amount of code to Linux. Recall that Linux is licensed under the GPLv2-only, and that the GPLv3 is incompatible with GPLv2-only.