In this matter it is important that who is interested lessor or lesses for the agreement.
1) Registration is mendatory is some states like Maharashtra irespective of period.
2) Agreement gives more protection to lessess than lessor.
3) Many people are under the misconception that eleven months agreement can be renewed any number of times to keep the tanent on leave and license basis and get possession quickly through rent control authorities instead of going to regular courts.
Please note that the moment the tanent has remained in the rented premises for more than one year which he can prove easily through mails, ration card , election card or even PAN card than the right of landord becomes limited for eviction.
Your repeated eleven months agreements have no meaning before the law of if the person has remained as a tanent for more than one year as a tanent in same premises .
Thereafter the right of the landlord to increase rent or get eviction are extremely limited.
4) Some times the rent control authorties pass orders for eviction in cases of more than one year tanency because the defense advocate do not fight the case properly.
5) In such cases aven after time lapse the cases can be got reveresed by revisional Courts or ultimately through the action at respective High Court.