The issue would be that the password is sent as plain text so a packet sniffer could potentially get the password. Depending on how much priviliges you give the account used to read the LDAP, could be a security risk.
See
http://support.microsoft.com/kb/321051 if you want to change your LDAP to use SSL, plus change your connection on Directory Sync to use port 636 instead of 389
Personally mine is set to use 389, and as our domain controller has outbound internet access I run the directory sync from it, negating the issue.