On the other hand leaving a computer on can wear down the always-spinning hard drive, unless you have one that goes into a sleep mode when not being used for a period of time. Dust on the computer's fan can make it an inefficient cooler. Leaving a computer unsupervised is not a wise decision because it only takes a real good series of surges during an on and off power surge session to damage parts of the computer. I leave my computer on all the time supported by a UPS and a sleep mode. The exceptions are if I plan to be away for a day or so or there is an approaching electrical storm.
Screen savers are an excellent feature for businesses to use to help sell their computers.
For the individual, the screen saver is a memory-gobbling piece of junk. If you are having trouble with program crashes you might consider not using screen savers except for special occasions. Screen savers can be very interesting and beautiful but they have some faults too. Well probably so, but the best way to protect your screen is to turn off the monitor at the surge bar when you plan to be away for a while. The monitor needs to be turned off any time you are away from the computer for a significant period of time.