Yeah, a repeater with the TST system is sometimes necessary. Mine will lose a sensor every once in a while. I haven't gotten the repeater yet, but probably will. Like was mentioned TST's major flaw is the very slow reporting of a lost sensor. I've had it happen and it really can take an hour to alert you. Waaaay too long IMO.
FWIW I just replaced the batteries in 4 out of 6 sensors after at least 2 years and the O-rings were all fine. I did clean them and lube them before reassembly.