Do they also limit the number of Es and Rs so you can't make dumb -> dumber?

Yeah, it's more than just that, though.

To win really seriously at Scrabble, you need bingos. (For those out of the loop, a Bingo is when you lay down all seven letters to form a word, earning you an additional 50 points above and beyond the normal score for the play.)

To do that, you need to hoard E's, R's and S's because they allow you to turn smaller words into bingos in multiple flexible ways. Thus allowing you to squeeze the bingo onto the board combined with the other words. E R and S are most flexible because the RE can be a prefix or a suffix to many words.

Example: "work" with ERS could become "reworks" or "workers" depending on the board layout.

Of course, if you hoard too much and make sucky plays the whole rest of the game, then a bingo isn't going to save you. It's all very strategic...