I did some calculations back in November for purchasing miners with and without WTT to find the break even points where i had earned back the money spent for the miner + WTT. This assumes WTT was at 1.70, and all crypto prices were calculated on 11-24-2017. But the math is still valid to show you a general idea of the benefits of buying WTT. Also, you are still being charged the discounted rate of $0.033 per kw/h while using WTT (instead of the standard ~$0.09 per kw/h) and this is factored into the monthly profit.
Miner | Rig Price | WTT (Price) | Hash Rate | Monthly Profit (No WTT) | Monthly Profit (WTT) | Break-Even
Antminer S9 (ASIC - BTC) $2,100 1456 ($2,475) 13.5 TH/s $517 $577 4 months / 8 months
Antminer L3+ (ASIC - LTC) $2,700 880 ($1,496) 500 MH/s $422 $458 6.4 months / 9.1 months
Alpha Miner 200 (GPU - ETH) $3,300 1540 ($2,618) 250 MH/s $387 $450 8.5 months / 13.1 months
Antminer D3 (ASIC - DASH) $2,200 1210 ($2,057) 15 GH/s $96 $145 23 months / 29 months
TLDR: Buy WTT for long term mining, you will get better returns in the long run, but short term the monthly profit difference is small.