According to Wikipedia's article on letter frequency, punctuation is the fourth most common thing after space, e and t. In Dvorak, the period and comma are on the top row, and with how frequent they are according to this, perhaps it's a good idea. I've tried Dvorak but at the time I wasn't nearly as critical about keyboard layouts as I am now. I'm currently using Colemak so it's hard for me to recall whether I enjoy having period and comma on the top row.
For those of you who use Dvorak, would you care to comment on the positioning of comma and period? Here are the points that interest me the most about the concept of having them on the top row:
1. Due to the high frequency of punctuation, having it on the top rather than the bottom makes them easier to reach. Since the top 12 most common letters make up 80% of total usage (again, according to Wikipedia) and can fit on the middle and top rows alone, having the punctuation with them can increase bottom row avoidance considerably.
2. If the period and comma are left on the bottom row and the top row is saturated after the home row, then there is an increase in row jumping if you need to go from the top row to punctuation.
3. "Punctuation" is a vague term however. How much do commas contribute? Is it mostly the period? If so, is it really worth giving up two spots on the top row for this?
4. Since a period or comma will always be followed by a space, having them on the bottom row isn't as harmful as you will get a break after reaching for them while you press the spacebar so you can get your hand back in position. This is a way of making the bottom row "less bad" as if you had letters on the bottom row instead, you would likely have to immediately hit another letter after pressing a letter on the bottom row so you don't get a break.