This is actually a tough call. Industry standard is primer plus 2 finish coats. Your guy claims that an additional coat will provide additional durability and will last longer. This may be true. Every additional coat is extra film build, but at some point you have reached a maximum film build and additional coats give you nothing. Where is that point? He's trying to sell you something that you may or may not need. He may actually believe in what he is trying to sell you or he may just want more money. I have often wrestled with the idea of offering people options for additional coats but never did it because I truly don't believe this would be giving the customer the best bang for their buck. If you have extra money, I would do it. Will it never need repainting again if you do it? It will probably at some point need to be repainted because it's outside and one additional coat is not going to make that much of a difference in the grand scheme. This is my opinion of course and every single painter has their own.

Thank you, paintguy. You confirm my suspicions. He is saying we probably will repaint down the line, but will only paint the lower portion, so making the upper last longer makes sense. I can't see only painting the lower, because color would not match!

Again, Thanks!

The painter says it's unlikely we will repaint the top portion down the road - this seems wrong to me.

Unless you are leaving in 8 to 10 years, it seems wrong to me also.The whole house will need to be re painted in about 10 years at best with any paint treatment and as paintguy said
" It will probably at some point need to be repainted because it's outside and one additional coat is not going to make that much of a difference in the grand scheme"

Primer and two finish coats are the ideal finish. Any topcoats beyond that is wasting money and paint. The top coat will suffer from environmental degrading regardless how many coats are beneath it.

It's best to use critical thinking before making such decisions.

Of course painting metal is totally different. 5 coats is the ultimate protection. Two primers, 2 finish coats and one clear coat.

Michael