Nowadays chargers all very smart, can cut off the charging power once it detects the batteries are fully charge, so I don't know if there's a functional advantage to overnight charging, compared to, say, the old days of trickle charging NiCds, which may take more than 10hrs.
Once a battery is fully charged, means it's fully charged, any additional charge will just be converted to heat. Also, Li-ion batteries have protection circuitry inside the battery to prevent overcharge so the battery will just "refuse" any additional charge once the circuit says it's full.
The only advantage I can think of is that your batteries are charging while you are sleeping so you don't have to wait.
I think what most of the people are talking about only relates to NiMH batteries. If your charger can switch to a trickle charge when the battery is full, then overnight charging will cram that extra few % charge into the batteries.
However, if the battery is a Li-ion, there really isn't any advantage because the charger will shutdown once it detects the battery is full. In fact, trickle charging Li-ion batteries will do more damage, but don't worry too much about it as the charger will protect you against this.