The big problem with those thermometers is that if the cable gets wet the thing won't work for weeks until it gets thoroughly dried out inside, if it ever works again at all.
If you think you must leave a thermometer in there try one of these:
This has a 12" stem so it will extend into your mash just fine when stuck through a hole in your lid. I have been using this same thermometer for years with my mash, though I only use it at the beginning of the mash to make sure I don't have any hot or cold spots. I found that I lose only 1-2 degrees during a 60 minute mash, so I don't bother to monitor it during the mash any more, just at the beginning and end.
Bugeater Brewing Company