The KEEP pool is just a buffer cache as DEFAULT and RECYCLE are.
If you do not size KEEP pool to hold all the data you want to keep in there blocks will age out the same way they do when in the DEFAULT or RECYCLE pools - Only difference is, competition for KEEP buffer cache datablocks is restricted to the objects you assign to it.
Disclaimer: Advice is provided to the best of my knowledge but no implicit or explicit warranties are provided. Since the advisor explicitly encourages testing any and all suggestions on a test non-production environment advisor should not held liable or responsible for any actions taken based on the given advice.
Thanks everyone for your helpful answers.
I did one test. Initially DB_KEEP_CACHE_SIZE was set as 0. I placed a table into the cache and when I checked the cache size it was 16M and the table was not into Default Cache.