It's an appropriate argument on a case by case basis - and different people will have different points of view.
For example, to have a complete set of compression suited Nostromo figures, it makes sense to re-use the same moulds and swap out the heads. Casting the moulds in different colours and paint schemes is only logical way to go about it.
Otherwise, there wouldn't be 5 of them (Kane, Ripley, Dallas, Amanda and the upcoming Lambert) to choose from. I'm all for them using the moulds wisely in this way.
Sometimes, however, it does appear that NECA mercilessly go out of their way to maximise return on their investment by re-using moulds with the most flimsy excuse. Recast the Alien in blue and call it a Kenner tribute figure, for example. ...or take the Stompy figure - an item that itself is a repurposing of the Alien figure - and give it a fresh head sculpt, release it and claim it's a Defiance comic book tribute.
In the end, it comes down to quality and while we're at it let's face it, such figures become recognised as variants. Other companies do it all the time, too. At least NECA are attempting to bring some validity to their re-purposing. Overall, I think it's a good thing but the sheer volume of content they are releasing certainly makes justifying buying all of their releases a difficult task.
At least we are spoiled for choice; and that's a good thing, right? So just pick and choose the ones you like. Who knows? Maybe one of these variants will become a favourite? I particularly like their Stompy figure of all their releases to date. 2nd place would be the Dog Alien.
They've released 5 of those. I have 2 and I'm happy with that.