Wow, a colossal waste of money. Space is far, far too vast and the time scale of human civilization far, far too small. To think of what other good uses that time and money could be spent on (that goes for SETI too)
Secondly, although it's hugely unlikely they'll find anything, in the event that they do it'll fundamentally change our perspective of who we are and where our place is in the universe. The outcome, while improbable, is so huge it's still worthwhile trying.
The same can be said about digging holes and filling them afterwards.
Yes, space is vast but once upon a time the ocean was too.