>>In which other situations did it have the same benefit?

in melees: two weapons sword + javelin (would probably work with Q too), sword+sword, sword+shield, two muskets, sword and club (club to throw back incoming items in case no shield is available)), club + any other item as baseball (apparently works with Q as well)
in settlement situations: as mentioned: the lanterin (or in the future maybe torchlight) + shovel when searching for a certain thing in the underground (However that is a special case, as the shovel/pickaxe use two hands already)

So by checking the played rounds (League), you find Brunnen(Well?), Schmelzendes Schloss (melting castle?), aerobatics, magma moles and very popular Heißes Eis (hot ice?). All scenarios that have eigher the windbag available, or the shield. In magma moles and dark mine (even though dark mine not so popular) would even make the light-source combo potentially becoming very sensible even in melee scenarios.

I am pretty sure right now also "knueppeln" is very very popular right now, which would not benefit from that feature, aproximately 80% of the scenarios at least offer items for which this feature would come in handy.
about the situations, that is of course much harder to determine. As far as I can recall, in Heißes Eis, any airbag combos are quite popular to shoot arount items or yourself, as well as club combos. In Schmelzendes Schloss, the Windbag is the most popular item for movement at least. furthermore there is a large quantity of these ice bridges(a mod for the metal bridge pack) available, which commonly are used while jumping (however I am to un-skilled to do that in a combo with that windbag thingy). And Brunnen everything is very chaotic, so 2 guns/ two swords would fit very well into that mess and the shield combo may become a popular counter agains that chaos.
So All in all, the most popular scenarios (not counting the knueppeln stuff) offer the requirements( the sensible items) and quite some situations.

Plus the benefit as mentioned, that it looks pretty neat, adds to the coolness factors.
Plus the impression (just my personal impression) that in games where it is not possible to wield two items are somewhat lost in past times and I usually missed that feature vs dual wielding became the standard. Most of RPGs seem to have implemented that feature now (started with Diablo 2 maybe, more than a decade ago). Another example was an online game called Biosfear, where they implemented that feature quite late, because a well supported request by the customers.

And as also already mentioned, we may like it or not, but we usually are compared to Terraria, Starbound&Co, which also includes that feature (I am not sure about Terraria, but at least there is a mod that implemented that).
Seeing Openclonk does not have it, is quite a lack.

It may be true that it does not feel more or less intuitive than the right-click throw & left-click use and Q, but according to my feeling playing Starbound, the controls there felt much more natural/intuitve, than this right-click left-click thing, which often happens in the worst situations, usually with the consqeuence of being killed, as it is quite a loss to throw away the sword to the enemy, leaving myself unarmed, instead of hitting him. this accident would not happen by chance when throwing would need a button pressed.

Then your nice short review of the linked threads shows that in 2009, MastroLindo actually wants to keep the two hand system at least for the popular combat items. The problem with quickly switching items would be solved in my approach, because you still would be able to switch arround quickly with your main item (while you still keep the shiled e.g. on your second slot).
The problem about the lack of two potential usage-buttons for things like magic staffs is also not possible to implement right now, as right click is reserved to throw an item away. So bringing up the argumentation of MastoLindo actually shows, that the current situation was not an improvement regarding his demands and wishes, but removed the posibillity of dual wield without bringing what he intended.

likewise Matthis argumentation about the advantages and disadvantages: he also brings up the shield situation as a benefit of dual wield (apparently really something quite striking!). Disadvantages are mainly referred to the very confusing hand situation (which is not the case with my proposal). Additionally the most striking argument to me is also the idea of having two potential buttons left for one item, also proposed by MastroLindo before. As said before, this feature is blocked by the right-click throwing. As the new controls offer many many more buttons now, I do not even see the need for that anymore. Multi-Use Items could simply be controlled by a somehow similar system used in knueppeln (different keyboard buttons for different functions).

I just scanned through the other links now shortly, but it appears to me, the most strinking reason of changing was the massive complexity and the confusion caused, by the two hands solution, where you could adjust the two hands freely. In my approach, right click would also be just a handy and nice minor feature. Just that I think the potential of wielding two items at the same time brings more opportunities than the idea of throwing away an item on right click. that it is not at all confusing when one item is "fixed" to right click, can be tested in Starbound very nicely. That right click throwing is confusing instead, can be seen by quite some awkward situations, when players throw there javelins and swords arround (which is not just me).