It's really just a matter of wear and tear. The standard wall-wart runs the UD at about 90% max power which really adds up (wear and tear) over time. For most users this equates to the UD sitting there idle most of the time, engine roaring away at 90% power, beating things up for no real reason other than to be ready to go at a moments notice. The standard wall-wart outputs approximately 13.5v while running an UD.
A VVPS by comparison allows a user to run an UD at a lower standby voltage between uses, say 10v, and then boost it up to whatever operating temp they want to vape at then lower it back down when done. This eliminates much of the wear and tear on the UD which has the potential to significantly increase it's lifespan. Alternatively one can leave the UD turned off between uses and then use the VVPS to boost up to operating temp in a matter of minutes (typically <5 min) and use as needed, this also reduces the wear and tear as the UD is running idle all the time.
So, choice in power supply is largely a matter of what works best for a given user. Some people are better off with a standard WW with no settings or anything to fiddle with, no forgetting to raise or lower the voltage when needed, etc. For these users it's probably worth the added wear and tear. Some users like to fine tune things, to tinker with them and/or want to really maximize the UDs lifespan.. for them the VVPS might be a better option.
Hope that helps..