Hi Bjarne,
Having a large transaction log shouldn't have any impact on normal operation of the database. The disadvantage of having a large transaction log file like this would be in the recovery time if they ever needed to rebuild their db file.
If the db file was ever damaged and they wanted to rebuild it using the transaction log, it would take significantly longer with this large log file then if they applied a log containing only transactions since the last full backup (to the backed up valid db file).
The choice of how often to truncate the transaction log (if at all) should be based on your recovery time requirements. More detail about designing a backup and recovery strategy is available here: http://dcx.sybase.com/index.html#sa160/en/dbadmin/da-new-backup.html
Hope this helps,
Mikel Rychliski