Do batteries have a shelf life? Yes. I have only seen one MTH battery leak, but this one was only 6 years old when I replaced it. After I saw that, whenever I buy a PS2 loco, one of the first things I do is install a BCR or SBR.
I agree with Oman. If you are going to sell it, install a new MTH battery. But if you do install a BCR, up the price.