In my opinion, thermal pads are better suited for components that need just that little extra bit of heat dissipation to perform adequately, i.e components that are < 10° C from being able to be passively cooled. Voltage regulators come to mind.
So in that instance, if the thermal pads are reasonably well installed, the components should be just fine, since we're working with much looser tolerances.
When you need active cooling, the manufacturing complexity increases to the point where the convenience doesn't really make sense, since you have to get it right, so thermal paste is the way to go.
Pads are preferred over paste for two main reasons: low assembly cost and ability to fill gaps. Both are very important. If you have an application using a pad, and it's flat enough to replace with (good) paste, you will always see a performance improvement.
I do not ever re-use pads. I have a used pad on my desk at work that shows an indent of the chip that was pressed into its other side... and you can read the part number and lot code laser marked into the chip. If the pad conforms to that but then can't relax back... it's never going to make good contact again. Full stop. (Granted, this was an extremely expensive thick gap-filler pad [TG-A9000, I think] for a particular application, and performance will vary.)
So in that instance, if the thermal pads are reasonably well installed, the components should be just fine, since we're working with much looser tolerances.
When you need active cooling, the manufacturing complexity increases to the point where the convenience doesn't really make sense, since you have to get it right, so thermal paste is the way to go.