Monsoon wrote: What would be wrong about learning the basic postures, movements and transitions, and then simply putting them together in a sequence you find personally pleasing?
Nothing at all, but you would have to relinquish aspects of lineage as far as claims go. You can't do it your own way, even if it is the real 13 postures, and then claim you are doing style X, Y or Z.
Does the sequence of postures in a form (any style) have a special significance?
Yes, there are at least 3 aspects to it in common forms and styles.
1, educational sequences akin to language development,
-----in this there is taught a usage pattern where strategy is informed in general in a non-application specific way. One learns the basics (13 postures) in the first parts of the form(s) and then one learns how to combine and mix them in different ways in the later parts. Sort of like learning an alphabet and then being taught words, then grammar and punctuation.
2, applications sequences for specific martial contexts
----- in this is taught methods for specific things, such as dealing with multiple enemies in specific situations etc, one can refer to the applications secrets regarding this, there is information about applications encoded into the form(s)
3, progressive skill development
----- in an archaic teaching method one would learn the first move of a form, learn it well by practicing it, learn the applications of it, and then learn the next move. One b y one, week by week one would slowly learn an entire form, and when one completed the form one would formally learn the names of the postures of the form. In this one would at the end of learning the form, be able to do it properly, there was no idea of learning a form at a beginner level and then learning it at an advanced level, that is a rather modern concept.
(4?)There is another potential reason, some claim that the form(s) are based upon meridian activation sequences, however most of this is modern speculation and the older teachings had extra qigong (not in the form sequences) with meridian pattern activation in them, along with meridian based striking methods built into the form(s)
I have also been taught formally that the form(s) have spiritual significance, this is not related to the meridians activation but has to do with hidden content and symbols. One might investigate the significance of the 7 stars to further explore this. The form is credited by many to incorporate and embody specific Taoist teachings, including but not limited to the I-Ching and the Bagua (the trigrams). I have read claims that the form(s) sequences are directly founded upon the I-ching, I have yet to receive formal confirmation of this, but it is worthy of consideration.
These are my thoughts.
being merely a student I cannot claim to have any expertise
I do
however practice both a form that is a traditional Yang method, and one I made from the 13 postures for myself and that draws from several taijiquan styles and systems.