> Is there any reason why changing the font should not > change the baseline also ?
Yes. Simple example: assume you want to set just a few words in a smaller size, but this short sequence of words happens to wrap at the end of the line. With which baseline spacing should the line be set? That of the larger font used in the rest of the paragraph, or that of the smaller font which happens to be "active" at the linebreak? Since this decision obviously depends on the context, it is better to keep the baseline spacing as an independent parameter, and leave the choice of both point size and baseline spacing up to the typographer. Like Clarke says, headers may require different leading than footnotes to look "good". Nevertheless, you can still write your own convenience macro to handle font size changes: .de size .ps \\$1 .vs \\$1+2 .. which works quite well in most circumstances (one exception is obviously the scenario described above). But still, the choice of what's appropriate depends on you.
