The content model of <summary> has been discussed on TEI-L (https://listserv.brown.edu/archives/cgi-bin/wa?A2=tei-l;c95232c6.1111) and some expressed the wish / the need to define a content model which is more flexible. Mentioned were the needs to allow substructures like paragraphs in case of a longer, but still summary-like description of content or decoration etc and to allow bibliographic references.
Right now, <summary> hasn't beed assigned to any content model at all.
I propose to assign it to a content model such as noteLike or msItemPart.
This seems to have been fixed in r12920 on 2014-06-30.