http://www.hobotraveler.com/blogger.html Stop spoiling your vacations, carrying a small umbrella allows you to enjoy your vacation.
I carried a great poncho that was custom made for me in Peru for years, when it was cold in the room, I could also use as blanket. It was great, I could also make into a tent. It was big enough to also cover my backpack. These are great for long walks, the problems is this, I do not make long walks in the rain, I do not need this, I take a taxi, or have some kid carry my backpack for one dollar.
The 1 dollar poncho is OK for for the 7 day tourists, but a 3-5 dollar umbrella is practical, cooler. Remember, the tropics is hot, your legs will get wet with a 1 dollar poncho.
Oh yea, I remember now, a one dollar poncho is good when you are on a motorcycle Taxi and you cannot use a umbrella.
Generally, there is the optimal, and that is an umbrella. There is a reason all the locals are carrying umbrellas.