Electronic and radio designers have their reasons for not using coin sockets.
Today, 20 years after this radio was designed, there are better battery holder designs on the marker and with current flash memory technology -- you don't need the battery!
The reason that I heard, long ago from the Far Eastern 3 mfg. in 1980s, was these were mobile HF radios -- and were deisgned so that the battery was not rolling around loose among ICs on a board.
I think a car battery is a pain to change - but like the 3 volt coin cell -- you only need to change it out once every 5 years or so.
Kenwood part numbers (East Coast Parts)
F20-0521-04 INSULATOR LITHIUM BATTERY (TM-2570A, TS-440S, TS140, others)
W09-0599-05 LITHIUM BATTERY (TM-732A, TS-440S, TS140, others)
Total for both parts was $ 6.88 USD in August 2005
East Coast Partshttp://www.kenwoodparts.com