> The only positive effect that the NDA may have for a producer is that it might prevent the already existing customers to redesign their products, because they might not bother to obtain similar NDA from other vendors.

That's a positive from their competitors requiring an NDA, not for they requiring one.

Anyway, large software sellers benefit a lot from stupid procedures from their clients that create a huge cost to start buying from a new supplier (like passing all suppliers through legal or having a management vote for them). If all hardware manufacturers require NDAs, they can force their buyers into creating similar procedures, closing the market for new entrants.