The EV mod is, in essense, not much more than a momentary switch between two wires. The Prius computer already knows about EV because the international models (if that term can be applied to a foreign car ) of the Prius come with an EV switch, while the US model does not. The Mod just turns the existing mode on/off.
As for harm to the battery, I would expect it will depend on how you use it. You probably already know it's a bad idea to let the battery to go completely dead. Treat EV the same way. If used with a modicum of common sense, I would expect you will not see any noticible difference.
an EV switch can be a good idea in some circumstances. if you frequently need to move the car short distances (you have one lane driveway and have to jockey with another car) but generally speaking the Prius does an excellent job of determining what mode you should be in.
Trust your car and it will go far... for much less of course!!