I don't get it, from the example you gave, seamstress requires a whole load of extra Perl code as well as a HTML template.
I would say that Seamstress requires a lot of Perl and nothing but id tags in the HTML (a standard thing) while in contrast Petal requires very little Perl but more programming in the XML/HTML. Looking at this another way, the learning curve for Seamstress is nothing but object-oriented Perl an object-oriented API for tree manipulation. The learning curve for Petal is Perl to the level of references and a mini-language for manipulating those references.
Seamstress can unroll tables in two ways:
using a row-oriented iterator method and it can also create tables using grid coordinates.
here is a reference to my query about alternating table rows in Petal... it is different from how Seamstress does it.
I have said time and again here that I do not like mini-languages. I spent enough time learning Perl and HTML so that I dont want to be concerned with hybrid technologies which are no more powerful but do require me to remember yet another set of rules and exceptions and limitations. To each his own, but keep me away from mini-languages personally.