I'm pretty sure most people using RDS will just stop using Iron Sight altogether.
However it's still good to have some BUIS (back up iron sight) in case your RDS fails
Using a RDS along with iron sight is called co-witnessing, you can search and read on the matter. But you don't HAVE to use iron sight in conjunction with the RDS.
Personally I prefer using the RDS alone.
Edit: an advantage of having both is to Zero both of them at different distance and using them accordingly
Originally Posted by Brian McIlmoyle
Real life comparison,
GBBR- bang bang -- Giggle
AEG-- merrrzip merrzip -- meh