It have no value and only for testing purpose of the network so the whatever amount of testnet coin you have are worthless. There are no any exchanger where you can exchange them as it may disappear when developer reset it.
AFAIK the resets in the past were only needed because people started to trade testnet coins. The current version, reset the difficulty to 0 if no block is found for 20 minutes, which happens from time to time, so it produces more blocks than the regular network. This also means its easy to get testnet coins.
