You do NOT NEED one. They are useful but I find them to be inaccurate in high pressure cooking (hot). Low and slow tends to be very accurate.
2013 Stumps Classic - Delivered 4/15/13
Large BGE - Rescued 2012
2010 WSM 22.5"
2010 Weber genesis gas grill
Rescued 22.5 Weber One-touch gold
BBQ Guru CyberQ
2011 Built UDS - Retired