RF Receiver Q/Q_N and I/I_N
You have to consider what is at the heart of most modern receiver chips and that is a mixer not dissimilar to the LT5512: -



Generalising a bit, the silicon inside will usually produce a balanced output (IF) and require at least the RF input or local oscillator to also be balanced (sometimes both).

The output stage is usually a differential pair of BJTs and only when you take the differential voltage from the collectors will you get the best quality signal.

Why is it a differential output stage you might ask? 

Because it's a balanced modulator and the simplest and (usually) most cost effective solution uses a differential pair to perform analogue multiplication of local oscillator and RF input. The bases are antiphase fed the RF input and the local oscillator can be fed to the shared emitter as a single ended input. More complex arrangements use a gilbert cell and these use diff signals on all three ports. There are also arrangements like below that only require two ports to be differential: -



This has a fair chance of matching the LT5512 and, although it only shows one collector as an output, for reasons given above both collectors will be used as outputs.

If you try and use only one collector single-ended you will get mixer artifacts that you don't want and you will seriously degrade receiver performance.

So why doesn't a chip like the LT5512 have a differential to single ended output converter you might ask. 

The answer is quite simply that if you are down converting to baseband with a baseband spectrum of several tens of MHz you would (or could) use an op-amp but, if you are not down converting to baseband and still have a seriously high IF frequency then an op-amp won't do the job and you are into balun territory then an amplifier.

So the chip (like the above) provides differential outputs.