HQPlayer is essentially takes a lot of the upsampling technology found in modern day DACs and does it on the PC. This provides far more flexibility than any one DAC can currently provide. Here is a list of some of the options, from their web page.
Resampling filters:
12 linear phase
7 minimum phase
3 impulse optimal
2 closed form
Dithers and noise-shapers:
4 dithers
4 noise shapers
Delta-Sigma conversion:
8 modulators
21 oversampling filters (64x - 1024x)
Direct rate conversions
Digital volume control
Convolution engine
Routing and mixing
These are options that a lot of audiophiles want to experiment with. They various options have various subtle (and sometimes not so subtle) differences. The options are quite different than the typical DSPs in MC.
It would not be practical for JRiver to try to duplicate the wide range of options that are implemented in HQPlayer. Roon ran into the same issues and built an interface between Roon and HQPlayer. A large number of Roon users use it in this way. They do this because the HQPlayer user interface is not very good. Playback is their strength, not the UI. MC has a big following because of its database and UI capabilities. Marrying it to the HQPlayer would provide an option for people who are really interested in the playback capability but want to continue to use the MC database and interface.
Think of HQPlayer as a extremely flexible DAC implementation in software. It has a pretty large following. Using MC as a front end might have a pretty good following, although Roon has established a pretty strong presence in that market.