And Collins (2000) argues for "keeping track of counts of arbitrary fragments within parse trees", which has indeed been carried out in Collins and Duffy (2002) who use exactly the same set of (all) tree fragments as proposed in Bod (1992).
the major innovations of DOP are: 1. the use of corpus fragments rather than grammar rules, 2. the use of arbitrarily large fragments rather than restricted ones Both have gained or are gaining wide usage, and are also becoming relevant for theoretical linguistics (see Bod et al.