Using same authoritative NSes multiple times in delegation

[Andrey G. Sergeev (AKA Andris)]

Greetings,


does the following setup violate any DNS RFCs or is it in the conflict 
with any best practices?

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[andris at strigidae ~]$ dig +nocmd +nocom +noque +nosta domain1.tld1. ns
domain1.tld1.           86400   IN      NS      ns1.domain1.tld1.
domain1.tld1.           86400   IN      NS      ns2.domain1.tld1.
domain1.tld1.           86400   IN      NS      ns1.domain2.tld2.
domain1.tld1.           86400   IN      NS      ns2.domain2.tld2.
domain1.tld1.           86400   IN      NS      ns1.domain3.tld3.
domain1.tld1.           86400   IN      NS      ns2.domain3.tld3.
ns1.domain1.tld1.       86400   IN      A       IP.Add.ress.1
ns2.domain1.tld1.       86400   IN      A       IP.Add.ress.2
                                                 ^^^^^^^^^^^^^
ns1.domain2.tld2.       86400   IN      A       IP.Add.ress.3
                                                 ^^^^^^^^^^^^^
ns2.domain2.tld2.       86400   IN      A       IP.Add.ress.4
ns1.domain3.tld3.       86400   IN      A       IP.Add.ress.2
                                                 ^^^^^^^^^^^^^
ns2.domain3.tld3.       86400   IN      A       IP.Add.ress.3
                                                 ^^^^^^^^^^^^^
----------------------------------------------------------------------

As we can see above, the ns2.domain1.tld1 / ns1.domain3.tld3 are 
actually the same physical host with the IP.Add.ress.2 and the 
ns1.domain2.tld2 / ns2.domain3.tld3 are actually the same machine with 
the IP.Add.ress.3.

What are the benefits of this setup?

Thanks in advance.


-- 

Yours sincerely,

Andrey G. Sergeev (AKA Andris)     http://www.andris.name/