Anti-aliased text can have disadvantages. If you want to optimize your image to make the file size as small as possible, anti-aliasing will make it more difficult, because it creates many additional colors where the edges are blended with the background. Anti-aliased text can only be created in True Color images, which require hundreds or thousands of colors. The greater the number of colors in the palette, the larger the file size will be.
You can reduce your color palette after creating the anti-aliased text, but the result will be less smoothly blended, and the file size, whether in 16 colors or 256 colors, will still be larger than the file size of two-color text which is not anti-aliased.
Regular text requires only two colors
Anti-alias text requires many colors
Anti-aliased text can also be problematic when you want to save the text object on a transparent background. The transparent background color is blended at the edges of the text, shifting it to a different color. When the image background becomes transparent, it leaves a halo effect where this impure color surrounds the text.
Anti-aliased text with transparent white background
The Halo Effect when the text above is dropped on a black web page
To avoid the halo effect you can either (1) use only regular, non-anti-aliased text when you need a transparent background, or (2) create your anti-aliased text on the same background color as your web page (in this case black,) and then place the rectangular graphic on the page without using a transparent background.