What do grubs eat?
Where do grubs cause the most damage?
I've never heard of treating beds for grubs.
Benefits of mulch:
Retain moisture (stones don't)
Reduce ground temps (stones don't)
Mimic nature (unless you're in the mountains or desert, stones don't)
Decomposition\modify soil conditions (stones don't)
Reduce weeds (stones don't)
Benefits of stones:
Keep window\glass companies busy (mulch doesn't)
Keeps blade manufacturers busy (mulch doesn't)
Keeps employees busy sharpening blades (mulch doesn't)
Increases insurance premiums (mulch doesn't)
I've said it before, I have multiple layers of fabric down in my plant holding area. I have weeds growing right through those multiple layers of fabric. Dirt and weed seeds that blow in between the rocks create a seed bed for weeds, so weeds can and do grow right on top of the fabric, as well as through it.
Fabric does not allow the proper decomposition of mulch. It does not modify the soil composition over time. You said that yourself, you've pulled 3" of mulch off beds that hadn't changed color or decomposed at all. That's exactly the point, fabric defeats the purpose of mulch.
And anyone applying 3" of mulch every year is ignorant. Should be an inch at most. At least by me.
“Every step we take towards making the State the Caretaker of our lives,
by that much we move toward making the State our Master.”
~Dwight D. Eisenhower