Everyone that's responded have said it pretty well, Cabosil doesn't alter the strength much from the neat resin.  But I do use Cabo for a few things.  For a fairing compound, I use about 25% cabo to 75% balloons by volume then mix in the epoxy until the desired consistency is achieved.  I reverse that when I want a hard putty that is stronger than just balloons.  It is rare that I use either one or the other without mixing them.  The reason is that cabosil alone in resin gives a gelatenous mix that drags the putty elastically when it's spread.  That is the putty stretches then rebounds, making it hard to spread accurately.  And of course it's hard as a rock...well it is rock actually sorta and dense.

Pure micro balloons is light and sandable but requires a huge amount of material to be thick enough not to sag.  So much so that it doesn't spread well.  It becomes very "bread dough-like" when it becomes thick and is again, very hard to spread.  But a little of both makes either one better than each one is by itself.

And you need to be carefull when you say "non-structural" because while the filler doesn't add much in the way of strength, the epoxy (or any structural resin) is pretty good in strength, just not nearly as good as a fiber reinforced resin.  And the filled resin used as a fillet can distribute loads over a much larger area than the plain resin can.

Dan Newland