The short answer is: it's complicated. While cushions labeled "all-weather" are designed to withstand the elements far better than standard indoor cushions, the term isn't a guarantee of indefinite, care-free outdoor living, especially in an urban environment.
"All-weather" typically means the cushion covers are made from solution-dyed acrylic, polyester, or vinyl fabrics that resist moisture, fading, and mildew. The filler is often a quick-drying foam. This makes them highly resistant to sun, rain, and stains. However, "resistant" is not the same as "immune."
In an urban setting, additional challenges exist. Pollution, soot, and bird droppings can soil fabrics more quickly. Limited airflow in compact balconies or courtyards can slow drying, promoting mildew even in resistant fabrics. Prolonged exposure to direct sun, even on UV-resistant fabric, will eventually cause fading.
For best results, consider these practices even with all-weather cushions: Use cushion covers for extra protection and easy washing. Store cushions in a dry place during extreme weather, long absences, or winter months. Clean them regularly with mild soap and water to remove urban grime. Ensure your furniture allows for drainage so cushions aren't sitting in pooled water.
In conclusion, you *can* leave all-weather cushions outside much more often than regular cushions, but proactive care and occasional storage will significantly extend their life and appearance. Think of them as durable, low-maintenance partners, not permanent, indestructible fixtures.