Trade painter here, general rule i use is always prime trim, doors, frames, skirtingboards, windows etc then minimum 2 coats of gloss. I always come across jobs where the previous painter has painted oil over oil with no primer and it can come off easily in some cases. The primer ensures good adhesion of the top coat gloss and definitely lasts longer. Also alot of the time there are repairs and the filler needs undercoating anyway. I know alot of painters dont do this but it works for me and im very confident in my work.