New Feature unusable Index-Partitions: is there any risk to use this feature?

Oracle-Version 11.2.0.3.PSU5 on aix 6.1

Problem description: we have a hugh table with partitions based on month for 10 years. For a new application we need a nonunique new local index but only for the last 12 months. Older partitions will be ignored by the application. My idea: I create an unusable locacl index and rebuild only the last 12 months partitions of the index, this saves a lot of space.

My Question: We have no experiances with such an index, is there any risk to work with unusable index partitions?

Many thanks for any answer,

Brigitte Türk