VTBD was going to be relegated to maintenance/cargo/parking when Survanabumhi opened but with the huge increase in traffic, due largely to the growth in the low cost sector, it had to be bought back online.
AirAsia had to move their operations over there to reduce congestion at VTBS which quickly reached its design capacity.
Whilst definitely not as nice as the new airport, I find a late night arrival around 11:30pm involves very little queuing. It also has a number of bus services into various areas of the city and a train station.
It's always worth ticking the box for "nearby airports" when doing a flight search involving Bangkok or you could miss out on some good deals.