It is becoming a popular option on a UPS to run in what is commonly called eco mode, where it stays in hot standby, running off the alternate bypass line as opposed to the inverter because of the increased efficiency. If the bypass should disappear, the UPS will transfer to the inverter, which is normally left running unloaded. As I understand it, Eaton says that their ESM mode is better than a normal eco mode because it will transfer to the inverter on power quality issues other than an outage.
It may also be of interest to you to note that some, but not all manufacturers put a true static switch at the output of their UPS, though the control system is usually shared.
The question really comes down to one of redundancy versus the need for up time reliability and statistically static switches have a higher up-time reliability than a UPS, but are designed this way because they are closer to the critical loads. In fact, in the oil and gas industry, what most people consider to be a UPS is referred to as a power supply and it is not called a UPS unless it has a complete redundant backup.
