For me it's being able to have different models.
I like multi-part plastics for the variety it allows you in your model building. I like to have my halberdiers looking like a uniform unit with mainly similar equipment and coloured clothing, BUT I like each model to be different in some small way.
I recently made a unit of 20 swordsmen with a pirate feel, so I used the cutlass sword from the militia sprue, but I cut several of that arm off at the wrist and then put the hand and sword onto all sorts of other arms - several from the Ed.6 soldiers box arms for example. Then I used various heads from the IG catachan set where they have bandanas.
Having said all that my favourite models are metal - most of them old Empire figures from the 90s.
I have still followed my theories on plastics above though, as where I have more than one or indeed several of the same figure, I again do some small conversion to individualise them.
Basically I don't look at it as an either/or choice: -
I am very enthusiastic about the new wave of non-GW plastics appearing as it allows people to get into our hobby at a lower price, and you can still make/convert character-level models from these sprues, so the famed Timmy's underprivilged mate Johnny can also have an army at a fraction of the GW price.
Timmy himself can also bulk out his army with these cheaper plastics and can have games at 3000, 4000 or 5000+ points more easily.
And at the same time anyone can get metal minis they happen to like from whatever source they want to integrate into their army as well.
