For a 2-4 day trip with the forecast not showing significant rain, and moderate temperatures you'll be fine. I've been using a Montane Lite-Speed (similar protection) on backpacking trips for protection against short thunderstorms, as well as a constant ~all-day light rain in temps around 45+ degrees. I get a little chilled, but as long as I keep a good pace, I'm fine.
The most important thing is protecting yourself from the winds to preserve your micro-climate. If you don't mind getting wet, a good hooded windshirt can work. However, if your insulative layer is down, long rest stops become complicated. I rarely stop to rest for much longer than 2-3 minutes, especially in rain, so I don't use my down jacket until I set up shelter for the night.
When you get into longer trips, then weather variability (temperature and rain intensity/duration) makes this less reliable. My experiences indicate that it won't work well for rain in cold temperatures, nor will it work well for a real storm.
Note that you'll have to learn how to read weather forecast models (or know someone that can), as typical forecasts are completely useless out here. A 30% chance of rain can equally mean a 95% chance of mid-day thunderstorm, or a 30% chance of significant storming all day. You'll be fine with a windshirt if the former, whereas, with the later, you'll definitely want to bring a real rain jacket.
I suppose you could do a longer trip without a rain jacket, but you may have to ride out longer duration storms under your selter if you start to get too cold. However, the weight you save would be completely negated by the extra emergency food.