For some plants it might help, think peppers, tomatoes etc. For salads, spinach etc I never use them. I do a lot of indirect sowing because pests and heat sometimes make it impossible to sow direct.
I have this model http://www.amazon.co.uk/Garland-Electric-Windowsill-Propagator-G51/dp/B000YA43HC I got it from my friends father, must be 20 years old. Works fine apart from the fact that the plastic tops are starting to fall apart. I also have this one (frighting expensive in the Uk, cheaper in Germany etc.) http://www.amazon.co.uk/Biogreen-HMTA060-120-GB-Aluminium-Thermostat/dp/B005910L5E/ref=sr_1_17?s=outdoors&ie=UTF8&qid=1323164714&sr=1-17 which I love. There are smaller heat mats and I like them better since you can put almost any tray you like on it. Another one I have (also present 15 years or so old)http://www.garden4less.co.uk/electric-heated-propagator-heat-grow-52cm.asp
Maybe an idea to ask around or look on ebay. If you have time on your hands you can build your own, just search on the net. But to come back to your question. You don’t need it but they are handy. Btw I’m in Italy (where it can be very cold!) so maybe UK readers can chip in.
