They are only used to distinguish letters that could otherwise be easily confused. In Helvetica those three letters are hardly distinguishable and thus almost impossible to read. What makes it even worse is that the letterspacing is too tight. Set in Verdana on the other hand, this text would still be legible at 8 points, a pain royale with most other fonts. Since we're dealing with a fast-paced medium, reading is equally hectic, so typography has to adjust to this speed.
Lines have to be shorter and leading has to be increased.
When you use sans serif fonts, you should increase this value even more. These fonts lack the horizontal guidance that those little serifs provide and thus help the eye follow the textflow. Individual letters have to be clearly distinguishable, so make sure you choose a font with the needed characteristics. Unfortunately one out of two Web browsers doesn't support letterspacing, but it doesn't hurt to provide those who do interpret CSS correctly with a looser spacing. Or, if you don't use CSS in your pages, at least specify fonts like Georgia for serifs or Verdana for sans first and only then Times or Arial.