Yes there is a benefit of earning proof-of-stake (POS) when the coins are in your own wallet ... download the wallet from:
https://cryptocointalk.com/topic/2144-emercoin-emc-information/
Skip down to downloads. After installing create a receiver address and send your coins from POLO to the address.
With proper redundant backups (throw one into your safety deposit box) you coins are probably safer than sitting on an exchange - esp if they gain in value and the exchange gets hacked (like what happened to Cryptsy although POLO I think is pretty tight - you just never know after Gox)
Staking with Emercoin gives approx. 6% interest/year. If that pays for the cost of running your own full node then you can justify staking with your own wallet. There's a POS guide at http://emercoin.com/MINING_GUIDE
If not then there are other options for staking. There are some online wallets such as https://emercoin.mintr.org/wallet and https://coinwallet.co/en/currency/EMC which offer to give you the POS interest.
This shares the same risk as with exchanges that the service will die or be dishonest and you lose your coins, but at least they offer to stake them for you and give you the interest. So do your research