Can we use the marked seams in combination with the Smart UV projection? Or is there a trick to achieve that?
"Smart UV Project" is ignored on edges with seams. The edges with seams are unwrapped along seams as if simply using "Unwrap" (but only for those edges -- the rest are smart UV unwrapped").
Using "Smart UV Project" and seams together would be advantageous where you'd like most of the mesh to unwrap based on the angle between faces, but would like some parts to be completely cut along each face (which is what marking seams is for).
If we "Smart UV Project" Suzanne, each edge is sliced when the angle between faces reaches more than 66 degrees (default angle setting for "Smart UV").
Where the rim of her ear meets the front of her ear forms an angle larger than 66 degrees, so the rim is unwrapped as it's own island.
...let's say we wanted a very specific texture mapped on her ear, but would like the rest to stay unwrapped as it is, then we could mark-seam each edge of her ear. In this way, the angle threshold (used to determine Smart Unwrap) is ignored and we're able to view each face separately.