With a solenoid valve installed in the sump overflow, one would be wise to install one in the fresh water supply also if you anticipate being gone more than a day or two.
If a power surge (lightning etc...) causes the circuit breaker that powers the aquarium to kick, or in the event of a long term power failure, or some other unforeseen pump failure, you would definitely want the drip to stop.
But man unless a we can find a cheaper alternative, this $hit could hurt the wallet big time. I plan on occasionally being gone for maybe a week at a time, so a drip system like this would be awesome for my fish.
There are some fairly good automatic feeders out there, and checking in a couple of times during the week isn't too much to ask of a friend. There is just too much that can go wrong when you expect someone not familiar with the hobby to do a water change though.
I already have my aquarium plumbed to a high tech Rubbermaid reservoir with a pump and a drain to the outside. Water changes are a breeze for me, but I cringe at the thought of what could happen if a friend left the wrong valve open by accident. This drip system is the answer for me, I just hope I don't have to shell out mega bucks to make it full proof.