I started wearing a abaya about 8 years ago. I was in souq asiri waiting for one of the material shops to open. It was around 4pm and most of the shops were about to open. Many of the indians working in the shops were sitting on the fountain- When I walked past I could feel them undressing me with their eyes. I popped into the first shop that was open and bought my first abaya. I put it on and walked out. I did not possess a abaya previously as I had had converted to Islam but was still sorting out my life as a muslim and one has to take step after step. So I think it was Allah's way of telling me then that it was time for my abaya.
I have since bought and had many abayas made. Some are simple and some for special occasions are a lot more decortive. Now I would not think of going out without my abaya. If I would go out now without an abaya I would feel naked. Sometimes I have dreams and then I realize I do not wear my abaya, it is more like a nightmare.
Anyway the point is wearing an abaya will cover you. Underneath you can wear simple outfits or even pyama (when I take my daughter to school). Before I wore abaya, I would have to get dressed to go to the supermarket, now I pop into my abaya and ready. Also we have many different fabrics, winter & summer.
So yes, the abaya is maybe more of a cultural thing, but it has a islamic root and that is to cover up.
Aisha-Taweela