I always announce "Speed brake flight detent" anytime I use it. For me it is just a crm matter and also to involve the PM in the action. Here is the reason: usually we use the speedbrake to catch up a descent path for being too high (i.e we have an extra total energy) or instructed by atc to expedite descent. In both cases our workload can increase rapidly especially during arrival/approach (starting to configure the aircraft, complying with speed or altitude restriction, ...) so when we remove our hand from the speedbrake lever to the mcp to set a new speed or altitude then setting the altimeter when passing the transition level we may forget to bring our hand back to the speedbrake lever. And since the PM is "actively" involved he/she will be more aware about the speedbrake position.
As for using speedbrake and opening thrust at the same time I can't see the reason unless during an emergency descent + need of using engine anti-ice at the same time. In normal operation, it is like braking and accelerating at the same time. You will waste fuel, energy and put an unnecessary load on the aircraft structure.