My first trip to Scotland was in 1990.
Had the same caddies for 4 rounds at Turnberry, Troon, Prestwick and Western Gailes.
After our last round, the caddies took us to their pub in Troon for a few pints and an early supper before we drove on to St. Andrews. Great fun.
At TOC, we had local member/caddies who we then accompanied to the "members clubhouse" after our round where we met a group of gents playing in an "Ireland/Scotland" club secretaries matches. One thing led to another and they invited us to join them the next day at Ladybank as a 4-some in their group were late no-shows.
We were all staying at the Rusack's and we also joined them for dinner and drinks. It was a great experience all because of the caddies.
We then met up with the same caddies two days later and played with them on the New course as their guests.
(We were supposed to play Scotscraig, but happily switched.)
I have since been back to Scotland more than 25 times and I never take a caddy now (except at a place like Castle Stuart where I usually am flying into Inverness en route to Dornoch). But, back then, it was an incredible introduction to how/why golf is just a more personal experience in the UK if that is what you seek or value.
Now, that said, I was with a group of 8 in September in Sandwich playing Royal St. George's, Royal Cinque Portes and Rye and we did take caddies at Sandwich and Deal for one round (played 36 at both) and it was helpful 1) on the body and 2) navigating parts of the course.
This same group is going to Liverpool next May and we plan on going "bare back" with just trollies...;-)
If US golfers are going over for the first time, then caddies can be a real treat especially if its a tour group situatiuon with 6-8 days of straight golf. Plus, as others have said, it's just not THAT much more expense relative to the entire trip.
More intrpid golfers who play the "next tier" of courses never take caddies.
It seems to be the domain of the "rota venues" plus the Kigsbarns and Castle Stuarts.