It is really interesting question, I will try to explain how I see it working:
First what we knew:
#1th If you have
Eternal MS then your eyesight doesn't deteriorate and
you don't get any bad effects from using it
#2th If you have
MS then your eyesight is deteriorating at rate depending on how much you uses MS jutsu and you get bad effect from using it.
#3th Senjuu DNA, especially Hashirama's is known to be
full of life which conatain: very good regeneration.
Now what we can assume from it:
If EMS user using MS jutsu
doesn't loose his eyesight , then MS user who posses Senjuu DNA
would still loose his eyesight but
at the same time Senjuu DNA could regenerate his eyesight, reverse process of loosing eyesight.
To make it easier:
EMS user uses MS jutsu -
0 badly effects to eyesight
MS user with Senjuu DNA uses MS jutsu - EXAMPLE:
5% damage to eyesight and at the same time 5 % regeneration of eyesight.
In other words MS user with Senjuu DNA can uses how many MS jutsu he desires(if he has chakra of course) without his eyesight deteriorating until Senjuu DNA's regeneration rate will be at equal or higher than MS's deteriorating rate. However If MS's deteriorating rate will be uprass Senjuu DNA's regeneration rate, then MS's deteriorating process will be slow down.
Another possibility is that Senjuu DNA's can't repair MS's user eyesight but instead only slow down this process
Assumptions:
Some MS jutsus as: Sussano, Amaterasu and Tsyukomi
can cause more strain for Uchiha's eyes than Kamui for example
Summary:
#1th Senjuu DNA repair any damage Obito's MS will recieve from using Kamui, if Senjuu DNA's regeneration/repairing eyesight rate is slower than MS's deteriorating rate then it only slow down damage to Obito's eyes
#2th Senjuu DNA can only slow down MS's loosing eyesight process, that why Obito loose his eyesight but very slowly
#3th Kamui made less strain of Uchiha's eyes than other MS jutsus
Lastly:
Question:
Why then would you need EMS if MS + Senjuu DNA give same effect ?
Answer:
EMS user Example : EMS user uses Sussano/Amaterasu/Tsyukomii/Kamui/KA or whatever - O damage to eyes
always !!!
MS user with Senjuu DNA:
Assumptions: Senjuu DNA's regeneration rate is 5% at using MS' jutsu each time
First exampale: MS user uses Kamui(let say 5% damage to eyes) so
before user's eyes are 100%,
after using Kamui they are:
100%-5%=95% but
at the same time Senjuu DNA repair Uchiha eyesight so:
95% +5%=100% (
AGAIN HEALTHY)
but
Second example: MS user uses Sussano, Tsyukomii, Amaterasu(let's assume 10 % damage to eyes) so
before user's eyes are 100%,
after using Sussano/Tsyukomii/Amaterasu: 100% -10%=90% but
at the same time Senjuu DNA repair Uchiha eyesight so:
90%+5%=95% (
STILL NOT HEALTHY)
CONCLUSION: EMS protect user from damage effects to eyes whatever he/she uses but MS user with Senjuu DNA can protect his eyesight from damage effects to eyes how long as SENJUU DNA REGENERATION OF EYESIGHT RATE IS HIGHER THAN MS's DETERIORATING RATE.
Hope you understands, if you have questions please ask me
Sorry for my bad english
What do you think about my theory, plassubile or no, please explain
I'll just give you a summary of what was answered:
