I think the wake behind a 747 a heavy as depicted and a Dash 8 (point of this topic) may differ slightly
Yeah that's fair enough -- it was the only cool picture I could find!
Saying it every call in a dash8 - yeah that's unnecessary.
Scenario where it is absolutely warranted: you land straight-in approach in a medium wake aircraft. Still air and unlikely your wake will blow away quickly. An ultralight late downwind turns right in behind you and their radio call gives you the impression they may not be aware of wake turbulence. You give them a quick reminder of a non-standard consideration.
Of course reminding everybody "Be careful mate your engine might fail" is stupid -- that's a standard consideration drummed in from first flights and standard for every flight. Wake turbulence is a non-standard environmental consideration that many people will never see. It is absolutely different than reminding people about normal aviation hazards.