[seer]

Since most of the weight of the rocket is fuel and payload, when landing it is very very light so even one Merlin engine, throttled to its minimum is way to much and the rocket has thrust to weight ratio bigger than one, so it cannot hover (the moment it slows down it starts going up again) thus the suicide burn - it has to cancel all of its velocities at the precise point of landing.

But even if that was not the case and it could throttle down enough, suicide burn is the most efficient way of landing a rocket (the least amount of fuel required) as it has very small gravity losses - the faster you slow down, the less you have to fight against gravity, so they would use it in most cases.

The amount of fuel left when going anywhere besides low earth orbit (or the international space station) is so big that they hardly have any to spare - the tyrany of the rocket equation and all that... You can test it all out yourself in Kerbal space program :-)