(The following all references musicals, though there are plays that fit the structures mentioned just as easily)
I think it depends on the intention of the play. Shows like A Chorus Line and 1776 build continually throughout the play, and the intensity of the buildup towards their climax is SEVERELY diminished by an intermission. The whole point of both shows is the time limit imposed by the story line (getting the show up, getting the Decleration ratified) and the intermission only breaks up the momentum.
On the other hands, if it's done properly, an intermission can do one of two things:
1)a first climax in the play taking you to a supreme moment of anticipation for what's to come (think Les Miserables and One Day More or Forum with Pseudolus about to have his neck chopped off). This technique also works structurally for some plays - going after I believe Greek comedies, look at the end of the first act of H2$ -- Finch is reaching higher and higher in the company, there's a problem that the love interest just discovered, AND you get the villain (Frump) swearing "I will return." It's a perfect setup for a break in the action, and a perfect false climax to the story as well.
2) be a natural dividing point in the story (think Into the Woods - how could there NOT be an intermission there? or West Side Story - after the Rumble, could we REALLY just go right to I Feel Pretty?)
Hi, Shirley Temple Pudding.