The reason why some people recommend not to do that is the same, why many people believe one shouldn´t stack batteries.
There is a tiny risk involved with unattended charging.
Almost all chargers cut power when batteries are full, some will switch off completely others will keep measuring the battery and do a very slow recharge if the battery over time discharges itself a bit.
IFFFF you have a VERY cheap charger AND a VERY cheap battery and IFFFF the auto cut off fails and IFFFF you keep on charging a battery WAYYYYYYY past it´s maximum and IFFFF you place the charger next to something easily flammable, the battery might heat up so much, you might get a fire while you sleep.
Rumors about exploding batteries are ........ well, let me show you this:
https://plus.google.com/+CaptainBlackbird/posts/Sv6sNhQuxPS
It´s in german, so if you don´t understand it, this guy has (in a controlled environment) tried anything possible to make the battery explode, from drowning it in hot water over recharging it with 2 amps while the cell was still wet, up to discharging it at high power right after.