High-entropy certificate serial numbers are a defense against hash collision attacks. The margin of security is reduced to the point that a brute force attack is twice as easy. If it was going to take you 10,000 years to brute-force it, now it takes you 5,000, both of which round to "impossible." If it was going to take you four weeks on EC2, now it takes two weeks, both of which round to "entirely too easy."