Not that anyone would ever mistake me for a query language guru, but that’s really part of the problem; I’m not a query language guru, because I’m a Web guru, and to a certain extent those two roles are incompatible.
The Web doesn’t do generic query, and it’s a better large scale distributed computing platform as a result. The cost of satisfying an arbitrary query is too large for most publishers to absorb, as they do when they internalize the cost of operating a public Web server and answer GET requests for free.
The Web does per-resource query, which is a far more tightly constrained form of query, if you can even call it that. It makes use of hypermedia to drive an application to the results of a query without the client needing to perform an explicit query. Think of a Facade in front of an actual query processor, where the user provides the arguments for the query, but has no visibility into the actual query being performed. FWIW, this isn’t an unfamiliar way of doing things, as it’s how millions of developers use SQL when authoring Web apps; a user enters “30000” in a form field, hits submit, and then some back-end CGI invokes “select name, salary from emp_sal where salary > 30000”.
I’m confident that SPARQL will be used primarily the same way SQL is used today, and that you won’t see many public SPARQL endpoints on the Web, just as you don’t see many SQL endpoints on the Web. There’s nothing wrong with that of course, but I think it’s important to keep our expectations in check; SPARQL is likely not going to enable new kinds of applications, nor help much with multi-agency data integration, nor do much else that doesn’t involve helping us with our triples behind the scenes.