Considering that charging over USB can take around 5 hours (to only 1-2 hours it would take from the wall mounted plug), can it be harmful for the smartphone battery if it's always charged from USB?
I don't see how the duration of the charge would relate to whether or not it's harmful to the smartphone. I have never seen any documentation that keeping a phone plugged in perpetually (whether plugged into USB, AC or DC) would cause any harm to the phone itself. An argument could be made that it's harmful to the battery.
Constant charging is not generally a problem for Lithium Ion Polymer batteries (which most new cell phone batteries are). It is in fact better to frequently charge them, and not let them fully discharge.
One factor that does degrade the battery life of all lithium Ion batteries is heat. The more heat it's exposed to the faster it will loose capacity, so I can see how keeping it plugged in may keep it warmer longer and theoretically degrade the battery life.
Here are a couple good resources discussing battery life: