It means that problem is not so big but 51% becomes 45% attack now if 6% miners use 0.8 without the limit.
That would be bad for a miner as working on blocks that will never reach 120 confirmations means no mining proceeds.
(i.e., it could cause an orphaned blocks, and they might build on that and get two blocks or more but they'll never end up with that being the longest chain again, mathematically).
There is a patch for those who wish to mine using v0.8. It will be released as v0.8.1 if I read correctly.