That's true, they don't have any value... But that doesn't mean you can mine them for free anymore...

I would give a tBTC loan to anybody who wants one, but for people i don't know i still request collateral so i get the tBTC back and i don't have to rent hashrate to mine new tBTC for my faucet out of my own pocket.

I know it kind of defeats the purpose, and i really wonder why there are people that insist on pushing the difficulty on the testnet so incredibly high by mining on the testnet with relatively new ASIC's... I'd love to mine on the testnet with my cpu, but if i did this i'd mine close to nothing  

I did a VERY raw calculation, based on prices i found on miningrigrentals:
You can rent an S9 with 16,5 Th hashrate for  0.00002/hour.
The diff is 10.5M
The block reward is 0.39 tBTC

So, if you run this S9 for a full day, it would mine:
 (((((16500000000000 * 0.39 * 600 * 65535) / 2^48) / 10500000) * 6 * 24) = 12.3 tBTC/day

A day of mining with this rented S9 would cost 0.00002 * 24 = 0.00048

So, mining 1 tBTC with rented hashrate would cost ~0.00004 BTC, NOT including the transaction fees, not including the fact that you'd end up with change on miningrigrentals, not including the time i'd need to setup everything, not paying for my efforts, not including the odds that i'm unlucky and hit less blocks than average,...