Backpack necessary for 3 weeks in Thailand?

I"m traveling to Thailand, a few days in Bangkok, a day in Sukhotai, 4-5 in Chiang Mai/Northern area, and then a week in the south (ko samui area). I plan on staying at places with sheets/private bathrooms/a/c. Do I need to get a backpack like everyone else, or is my big travel bag with a shoulder strap good enough for the trek to/fro from bus/train/air stations and the lodging area.