Are demolition extras a legitimate cost or a low-bid counter measure?

In an age when clients are expected (and, in some areas, legally obliged) to disclose precisely what the unsuspecting demolition contractor is letting himself in for, are demolition extras a legitimate additional cost?   Do they highlight the ignorance of clients and their project managers?   Or are they a sign of something a little more sinister?   Are demolition contractors attempting to offset the demand for low bids by bidding cheap to win the work and then ramping up the extras to achieve something approaching a profit?