Sometimes they're good for "need ASIC-like results in a low-volume design" but that's more a special case of what you give, and of course the 'upgradable glue logic' case also is in play.

I think the killer application would be to have programatically reconfigurable FPGAs that reconfigure on the fly to fit the needs, which has been discussed in various overhyped ways in the past, but I don't think the tooling is there. This effort might or might not make that possible -- imagine a chip where, say, the regex you're going to use to scan 1GB of data could be turned into an FPGA and then it just shovels the whole thing through a "HW" FIFO that detects matches much faster than a processor could. Nifty... but is it worth the effort to do?