I've always gone with the idea that Deckard is a replicant and the Unicorn is a symbol of that (his dream is part of his memory implant - so outsiders know what he dreams of).
As for the dove - it is the replicant's way of showing his own humanity - and why he should be treated as human. He has the power to destroy the dove but allows his human side - his compassion - to make the choice to let it go. The replicant knows his time is up and destroying the dove will achieve nothing..
Just my own interpretation
