statuscache -- why isn't this enabled by default?

[Patrick Goetz]

Again, reading through the documentation (now supplemented by the 
*secret* documentation -- thanks, Nic), I noticed:

    statuscache: 0

       Enable/disable the imap status cache.

With this description under database-formats:
----------------------------
STATUS cache (statuscache.db)
This database caches IMAP STATUS information resulting in less I/O when 
the STATUS information hasn't changed (mailbox and \Seen state 
unchanged). The database is indexed by mailbox name + userid and each 
data record contains the database version number, a bitmask of the 
stored status items, the mtime, inode, and size of the cyrus.index file 
at the time the record was written, the total number of messages in the 
mailbox, the number of recent messages, the next UID value, the mailbox 
UID validity value, the number of unseen messages, and the highest 
modification sequence in the mailbox. The format of each record is as 
follows:
----------------------------

It's seems like a no-brainer to enable this, particular for sites with a 
lot of I/O.  Why isn't this enabled by default?  Are there some 
drawbacks not revealed in the documentation?