If due to some misadventure the index gets out of sync with the db, if there is no lifetime, the error will never be corrected. You should recreate the index every day or so - use something like ToastCache for EEPC/EEM v5 | Simon Says - Musings of a Mid-Tier CTO to force it to rebuild at your convenience.
I understand the need of re-creating index file thanks to you.
And more question.
Are index files useful for the performance of client machine sync processing to EEM server?