Compact body of Pentax RS1500 makes it much easier to carry it with you all day. 157g
This camera is so light that you can comfortably carry it around all day. Environmental Sealings
For a camera that you carry everywhere with you, being weather proofed is a big advantage. You will have one less thing to worry about.
3.9X Optical Zoom
Having a camera with higher zoom ratio would be more practical. Small 1/2.3" (6.17 x 4.55 mm) sensor
Pentax RS1500's small sensor struggles in low light situations compared to larger sensor. No RAW shooting capability
RAW files contains much more color information compared to JPEGs, and gives you more room for post processing without significant quality loss. They provide wider dynamic range and option to recover shadow and highlight clipping to some degree.