When I include a Google font in my website and I just include the Normal 400 weight, I am still being able to use the bold, italic, etc weight although it hasn’t been really included.
This is a fiddle: http://jsfiddle.net/9nYZb/
<link href='http://fonts.googleapis.com/css?family=PT+Serif' rel='stylesheet' type='text/css'>
vs.
<link href='http://fonts.googleapis.com/css?family=PT+Serif:400,700,400italic' rel='stylesheet' type='text/css'>
Why is this?
The browser will emulate the bold and italic fonts by simply making the standard font thicker or slanted.
If you compare the two, you will see that the one with the bold and italic fonts included looks better.