Yes Muggle-techonology doesn't work at Hogwarts, but outside the castle, it sure would have helped Potter, Ron and Hermione at many instances, isn't it? Well,
Using the telephone to call Ron when he left for Bill and Fleur's? Using the internet to look up the daily news, search about Horcruxes? Using satellite TV to broadcast Potterwatch? Using mobile phone to text, tweet updates among the anti-Voldemort movement? Using motion sensors, motion alarms to help detect movements around the tent at night?
And so on.