I would say you should look at running back tests against real market data to obtain a better picture of performance on a strategy. The testnet is great for developers to test functionality against though.
I must say I find the concept of the Bitmex test site pretty weird. Why it doesn't mirror the site's real trading I have no idea.
The other sites usually do follow real coins with no real money on the line. I can't remember which one, but one of them just mirrors all of Poloniex so you get all the corresponding movements to ruin yourself in safety.
It might be design in users side
solely purpose on its trading features look and feel
If its mirrored in actual price the newb users might get an early disappointments
Honestly, started in normal trading before jumping unto margin ones
I have to use testnet to simulate trading features in bitmex as it really has a big contribution when engaging with them in real trades