The primary reason for leaving solid state on all of the time is because transistors will not degrade nearly as quickly as will tubes from constant use and solid state gear takes much longer to warm up than tube gear. The primary downside to leaving solid state on all of the time is that this exposes the gear to more mishaps, such as powerline surges and brownout (undervoltage can be particularly hard on solid state gear) and weather-related mishap. Also, while transistors have a much longer life than do tubes, they are not nearly as easy to replace, and it is sometimes the case that there are no replacement parts available many years later when they do need replacement. By contrast, most 80 and 90 year old tube gear is easily serviceable.