There is no way anyone would just let the easily obtainable oil sit there. Alternate methods of energy are gradually making progress but not fast enough. The tar sands disperse the coveted natural resource of oil to places outside the Middle East and can lower prices and stimulate growth and the economy.
Insofar as global warming and environmental preservation are concerned the exploitation of these resources is deleterious to the local ecology and not exacerbating the increase of anthropogenic greenhouse gasses. But it will theoretically reduce the cost of oil due to supply increase and could stimulate the economy in some small measure consequentially.
Ethically it ought not be the case that humans increase those gasses which increase the thermal inertia of the planetary system due to the consequences which are highly likely. And it ought not be the case that the human desire should trump the persistence of an ecology of an entire region.