There are times when a voiceover can make an ad and when it can truly kill it. A really nice example of a TVC whose voice over gives it real soul is the Range Rover ad. It really grabs you and involves you in an engaging story.
On the other hand, something that really kills the creativity of the commercial is the voiceover for the commercial for the new Toyota Camry ad. It is evident that the footage is from some brand film, probably shot abroad, this footage in itself would have done enough to sell the ad along with the last line of the voiceover.
A voice over only adds value when it is able to connect to what is being shown, if it adds to the story that is being told, if it doesn’t perhaps just a soundtrack or a dramatic pause may suffice.