I've experienced the same issue with this and I think that setting the bin log mode to MIXED has the following implications:
Logging will use STATEMENT mode under most circumstances and then switches to ROW mode for other circumstances as described in:
http://dev.mysql.com/doc/refman/5.1/en/binary-log-mixed.html
STATEMENT mode requires much less storage space as it will literally log the statement which was executed to affect the database.
ROW mode takes more space as it logs the changes that have occured to row data.
So if you ran "UPDATE mdl_user SET lastname='Smith'" on a moodle database (don't run this!) then STATEMENT mode would literally just log the statement you have executed (obviously this will not take up much space in the bin log). However, if you ran the same SQL in ROW mode then it would have to log the change of lastname to Smith for every single row in your database. So you can see how STATEMENT row is better in terms of bin log size efficiency.
MIXED mode is the best option to use for Moodle 2 as it will keep the bin log size as low as possible by using STATEMENT logging where ever possible.
I hope this helps people understand the concequences of having a MIXED bin log mode.
Guy Thomas - OvernetData