Life safety generators, emergency fire pumps, and the like have block heaters on 24/7/365. Many of them do use thermostats, but they are not allowed to cool off completely unless taken out of service.
My 97 Dodge Cummins gets plugged in about October, and unplugged about April. Mostly because the beast has a huge cooling system, and if I'm not driving for an hour, I ain't getting any heat! I also block off the air flow at the grill.
You most certainly will not harm one by leaving it plugged up, but I have always contended that you can 'spoil' one by plugging them in, meaning they will start cold, but will whine like your teenager when you get them up for school on a cold morning!
George