Are there any tangible benefits for coupling an NNID (3DS/Wii U) to a Nintendo Account (Switch)? Could people unlink them, or are they permanently tied?
What's this about hijacking text messages? What does that have to do with the Nintendo Network? I don't understand.
It has to do with 2 factor authentication/multi-factor authentication (2FA/MFA).
2FA/MFA are methods to increase security on accounts. It requires new logins to not only provide the correct username + password combination, but also sends a temporary code to a mobile phone. Those temporary codes are time-sensitive and need to be entered within a certain timeframe (30 seconds for example).
Where the SMS/text message hijacking comes into place is at this step.
SMS/text messages can be spoofed (faked). If someone has access to your Account, they can perhaps figure out your phone number. (Maybe it's in the NNID account info they've got access to, or maybe they just try to use your password across many services to see if an account re-uses that password.)
From there, using spoofing, they could still intercept the 2FA/MFA temporary code, and use that to compromise your Switch Account.
It's more hoops to jump through, yes, but this is why services are pushing clients away from SMS/text-based 2FA, and towards using an app like Authy/Google Authenticator/Microsoft Authenticator. Those should encrypt the temporary code, making it more difficult for outsiders to crack the unique code within the 30 second timeframe.
TL;DR: texting/SMS is thought of as a less secure method of communication than using end-to-end encrypted methods. Thus, 2FA login codes are phasing out SMS and favouring encrypted Apps.