It is sure a tricky case if the perfect solution is the goal…
But perfection doesn’t exist (sadly 
), especially when Last.fm data quality is concerned.
Scrobbling all primary artists instead of album artists only adds mess to the growing mess that Last.fm data is, but that is not your concern.
However, Roon current strict behaviour adds incoherence to the data of users who scrobbles for a long time or simply a lot before using Roon.
I’m only scrobbling for a year now (with dedicated plugin in foobar2000, and now with Roon in trial) but I’ve paid a lot of attention to the data scrobbled to have decent results.
Since the plugin I used allow customization of the data scrobbled for artist, album and title, I created a simple rule for artist value :
If the track artist metadata count is greater than one I use album artist metadata for the artist scrobbling value.
The only exception is when album artist is a standardized (in my library tags) generic value like Various Artists. In this case I scrobble track artist(s).
In my view, a premium product like Roon should allow at least allow to choose the source for artist data scrobble, either track artist(s) or album artist.
The exception management I described (or a more advanced one) would be an even better solution, maybe later.
Let me know what you (other users or the Roon team) think of my suggestions.
Thanks.
Wouldn’t be better to continue this discussion in the Feature Request section ?