Unlike the more familiar relational model, DAV does not define tables or schema for resources, hence there is no guarentee that all properties will be defined for all resources. Moreover, DAV allows an client to store arbitrary properties on arbitrary resources. Therefore DASL must support queries that use properties that are not defined on all resources in the scope. If such a query failed, there would be no way to locate the desired resources. We do not expect the first version of the DASL protocol to meet this requirements.
More powerful comparisons are useful when strings encode structured data such as times or lists.
Note that these are constraints on what the protocol must define, not on what servers must neccessarily implement. Support for structured value criteria is needed, for example, to locate resources locked in a certain manner by a certain principal. The working group concensus is that this feature, while undeniably very useful, is so difficult to define that it is better for DASL to proceed than attempt to define it. Also, there is much activity in the W3C to define an XML query language, and it was felt better to wait for this to complete than to define a competing standard. Different cultures define different rules for string comparison, e.g. for collating sequence and for significance of diacritics.