SD has a built-in feature to convert the tracks to wav's.
I use SD, and I've used the internal bounce to audio...and it works fine.
However, I discovered one issue.
If you have say....SD MIDI tracks....and you have some synth MIDI tracks...and you want to bounce everything out to audio...the tracks have a minor synchronization issue, because the SD bounce and the synth bounce are not using the same mechanism to do the bounces.
It's minor...but it can be audible enough.
So instead I just record out with all the MIDI tracks in real time, and the sync-lock is perfect.
The telltale sign for me was a kind of swirly sound in the cymbals. Just not good enough. It sounds better to my ears if I pre-render the drum tracks.
I think you are right, and it may not be always.
Playing back lots of samples in real-time can be a system strain...or simply the way the sampler is playing them back, as it's about triggering samples, and not playing existing audio tracks...but it's more than just the computer processing.
It may not be an issue for folks...but let's say down the road, you get a new computer...you install new plugs...you change up you DAW...etc..
...by leaving them as MIDI....all that shit has to come together perfectly down the road should ever need it. If for instance you no longer have a particular plug or the sample pack, or you can't use it with your new systems etc...your MIDI tracks will need to be recreated, or at least you will have to reassign shit and maybe use different plugs or sample...etc.
I use to do part MIDI/part Audio...and once I pulled up some old stuff, and realized that I was missing some of the patches/settings...and now everything sounded different.
Once it's exported to audio...it's an audio track and it will always sound the same.
Of course you want to wait until you've made all your decisions about the MIDI track...etc...but once you are set, there's no benefit to leaving it MIDI, and it actually drags down your DAW since you have to keep using the plugs.
Not to mention...I just like having all my tracks as audio since they are committed. I mean, if you have 20 audio tracks which are committed, what's the point of keeping just the drum tracks as MIDI, if you are at the state of working up your mix?
I also found that to my ears...the audio sounds better when I dump it out from MIDI to my tape deck, than it did when I was just playing back MIDI samples. It just gets a fatter sound, especially the drums.