You're welcome! I last looked at Mercury about 3 years ago, and I vaguely remember partial instantiation being documented as an eternal low-priority future feature. Maybe there's been progress... or maybe they'll say this is less useful than Prolog weenies like to claim ;-)
Out of interest, in what sort of function do you talk to Mercury users and developers if you don't use it yourself?
Oh I just bring attention to tech that people might be interested in. You're the first to mention a drawback. So, I might just ask one of them if there's a list of thing people like about Prolog hard or non-existent in Mercury. Next time I'll be better informed if anyone asks. That's all.
I do have a slight tie-in to my field where I keep logic systems in my peripheral in case they become useful for (a) executable specifications, (b) connections to provers like Milawa, (c) easier formal verification, or (d) commercial deployment in a good one that almost skips the coding phase by keeping code close to specs. Mission Critical IT claims to use Mercury for (a) and (d) with a finance developer linked here a while back using Prolog for whole app. Got easy extensions and fixes as result. So, these conversations might have benefit later.
Out of interest, in what sort of function do you talk to Mercury users and developers if you don't use it yourself?