Is it necessary? No. Is it a very good idea? Yes.
It's all about risk. How much money are you willing to risk by not conducting a sea trial? I guess there's some point where spending all that money to de-winterize and re-winterize doesn't make sense. What that point is, that's your call. My 1st watercraft, a PWC, I bought it brand new at a boat show, no sea trial (but full 1 year warranty). Second boat, bought from a private party, no sea trial, but ran the engines on the muffs for a good long time, and looked over everything I could. I was only spending $6,000, so I was willing to risk that much. Current boat, bought it new at a boat show, full 5 year warranty on everything. No sea trial. I figure I have a 5 year sea trial going.
Plus, in my area, a sea trial is an all day event, since the closest water is about an hour away. Sea trials around here are kinda hard.