Nar Shaddaa has a bunch of quest at lvl 30 that are a bit hidden including two heroics. I found them when going for the datacrons on the empire side. Never saw another soul in there.
Are you kidding? I play for Republic, and travelling to get datacrons on the Imperial side on Nar Shaddaa always results in mass-pvp. At first there's no one around, and suddenly arrives a party of Imperials and some Republic players...
There's a recent example - yesterday i was trying to get Yellow Matrix Shard and was confronted by a Sniper (i play gunslinger myself), which resulted in a long duel. I got killed twice, he got killed three times. Then we just silently stopped dueling and jumped for the datacron
On the topic: Bonus Series provide extra exp and money, and also through them you can discover some more locations on the planet, which you may or may not saw while doing main quest.