>I don't presume to speak for Cristóbal at all. _I_ think the idea is that the user doesn't not have to _outside_ of the grid to add. <

If this were truly desirable, we wouldn't have toolbars. :)

>If this button is available in each row

That seems wasteful of space, to me. In general, I have problems fitting information horizontally in grids (since I don't believe in horizontal scrollbars).

>then they wouldn't even have to go to the end of the grid to add a new line.

That's indeed a benefit. In my own design (with a single Add button), they don't have to go to the end, either.

But hey, there are all kinds of ways to skin the UI cat, and my opinions are just that--opinions. :)