Hi!
Actually I think that having "no value" for the end date qualifier probably means that it has not ended yet. There is no other way to
But that's what no end date also means, in 99% cases where there's start date and no end date. Let's see https://www.wikidata.org/wiki/Q30#P35 - does it say that we have no idea if Barack Obama is still the US president (same for P6) and nobody bothered to check? I don't think so. I mean, maybe that was the original idea, but are we going to go and fix all start/end pairs now and add novalues to them? Are people editing Wikidata even aware this is what they should be doing - in case it is what they should be doing? I think in this case the common usage and the intent of the editor would be in 99% of cases that start date and no end date means current event and not "we have no idea if it's still current or not". At least for something like P582. I admit, for some others the meaning may be different - i.e., if there's neither P580 nor P582 then the above reasoning does not apply. But then we by default assume it's current (unless it has P585) so the outcome is essentially the same.
Other qualifiers I could imagine where an explicit "no value" would make sense is P678, I guess.
OK, here I don't know much about what it means, so I just accept your point.
In references it might make sense to state explicitly that the source does not have an issue number or an ISSN, etc., in order for example to allow cleanup of references and to mark the cases where a reference does not have a given value from those cases where it is merely incomplete.
Here though again the same as above - usually when you add something that is expected to have issue number but it's not there, it's either somevalue (means, we don't know what the issue is, but it was an issue) or somehow it's the exception and it has no issue. Only actual usage of novalue I found in refs so far was confused usage of refs instead of qualifiers (pretty soon - ~couple of weeks - we'll have full recent dump loaded in the lab machine and we could answer this with real certainty, right now it's like 80% certainty :).
I don't have superstrong arguments as you see (I would have much stronger arguments for "unknown value"), but I would prefer not to forbid "no value" in those cases explicitly, because it might be useful and it is already there.
For qualifiers, I can see now there might be cases where it is useful, still not on references. But I think maybe not forbidding as such but having the guideline on what is considered the Right Thing and then if there's an exception than the editors can use their own judgement.