Deep Voice Guy from Rohde & Schwarz has my favorite explanation:
These days Rigol and Siglent (and SignalHound -- I have a SM200B that I love) offer this capability at a much lower price. Of course, I have yet to see an implementation from the economy players that matches the quality of the implementation from Keysight or R&S. Color grading, resolution, update smoothness, ability to really crank the juice on the spectrogram, triggers, etc are all much better from the Big Players. Still, these are exciting times, and it might well make sense to spring for an RTSA!
That said, once a spectrum analyzer has a powerful digital IF, typically they don't expose IF Out anymore because they want to steer you towards their own software solutions. Analog IF Out is good for time correlating signals on an oscilloscope, using a SA as a filter / downconverter, digitizing and steering signals into GNU Radio, etc. So, as always, there are tradeoffs.