Pat,
Strategically, they can be used to require certain shots in absence of connected shot stragegy, and it helps, knowing multiple tees make it fairly equal shot for all players. Similarly, a modern use of Par 3 holes is to cross Env Senitive areas that cannot be touched, and at least the forced carries come with the ball on a tee, and some distance control via multiple tees.
I agree Par 3 holes are the easiest holes to put in a routing and that sometimes they come last as connectors. In the main, it takes a long time and much trial and error to produce a final routing. I cannot recall too many instances where I consciously thought of a par 3 as a "connector." I plan on using 4 (I have never done a course with 3, but have one with five and one with six).
In fact, I actually visualize a short 4 or some other long hole as a better "connector" over tough property. I have no qualms about an uphill, blind approach on a short 4, compared to other holes where the uphill shot often becomes an unpleasant one, especially for shorter hitters, and after a bum tee shot.