Google URL Shortener Opened To the Public
It's about keyspace.
Given 4 bytes of [a-zA-Z0-9] gives you 14,776,336 possible combinations while [a-z0-9] only gives you a mere 1,679,616 possible combinations.
Assuming they'll eventually up the number of bytes up to six (ie. 4 to 6 bytes), you'll get 57,731,144,752 combinations case sensitive compared to just 2,238,928,128 case insensitive.
What Is the Oldest Code Written Still Running?
Memory unused is memory wasted. Why should you allow your precious DDR3 to just sit there and not be utilized when you could avoid time-consuming disk I/O by buffering data to RAM?