For the record, I hate
that "feature". What it means is that if you accidentally leave a field that is in the SELECT out of the GROUP BY, you'll introduce a join. Depending on the query, this join can be very large and very poorly behaved. Particularly if you've left joined to a large table.
When I worked with Sybase I think I made this mistake every few months (with the left out field from a very large table), and inevitably didn't notice until my query had been grinding away for half an hour and the database was feeling the pressure. The database it usually happened on was used for production purposes, but luckily the consequences were only internally visible, and very few people were affected. Still it was my second greatest annoyance with Sybase. (They've since fixed my greatest annoyance at the time, which was page level locking.)