Breather holes have the advantage that you can treat the insides of more tubes with an anti-corrosion product. This probably isn't necessary on most Thorn frames, which are usually treated before delivery.
And, if you wish, you can "seal" them afterwards to slow/stop water entry. I generally use window sealant, as I have this around for other jobs.
I currently treat frames I acquire (usually second-hand) with AC50 spray, then block the holes in small tubes with window sealant after leaving the frame/forks to dry for several days.
Seat tubes and head tubes/steerers need occasional checking, as water can get in around the seatpost or stem (in my experience, more often with Aheadset type steerers), but these two areas are relatively easy to access and treat.