First, and in my mind most importantly, it would dramatically extend the length of wars. Fighting guerrillas is already very tough, and the proposal to severely curtail attacks on mixed targets would make it even more difficult. This (combined with the often loose organizational structure found in many guerrilla groups) would make negotiated settlements or surrenders very unlikely. The wars can (and do) continue across many generations. This isn't good for civilians.
Second, it would reinforce the already existing impulse to mix military and civilian targets. If you give extra-special protection to mixed targets, you are directly incentivizing the co-location of military and civilian targets. This will tend to be very bad for civilians in the long run unless you completely ban attacks on mixed targets, in which case you lead directly into the indefinite war scenario I outlined above.
Third, it gives no incentive to engage with combatants on the other side. Why fight soldiers who can fight back? You can attack civilians exclusively and retreat to your mixed civilian/military sites with impunity.
Fourth, it gives state actors an even greater incentive to engage in proxy wars with guerrillas as their soldiers. Warfare with non-state guerrillas is bad enough, but thought not to be as dangerous because they don't have regular access to the weapons that only states can produce. If you combine them with state sponsorship, they become much more dangerous to civilians because they have access to the weaponry of a state war but fail to respect non-combatant immunity.
Fifth, and perhaps as important as the first, it directly plays into the propaganda war that the guerrillas want to play. It not only reinforces the tactical need for safety, it helps the strategic goal of propaganda--if mixed targets are offered heightened protection when used by guerrillas as opposed to regular military then any civilian deaths (or combatant deaths that can be reported as civilian deaths) have even greater propaganda impact.
This is a definite "read it in full" post. It has some of the sharpest analysis of the dangers that lie in changing the international law status quo I've seen anywhere.