Originally Posted by ThatBritBloke

Using extenders increases the Q-Factor of the bicycle. In other words, widens your stance on the pedals. Riders requiring minimal adjustment, ie a couple of millimeters or so - and millimeters do count - can use pedal-washers. Increasing your Q-factor reduces cornering clearance while pedalling, although this isn't normally a major issue unless you regularly race street crits and stuff like that.

Conversely, some riders require a reduced Q-factor. This isn't so easy and if it is a consideration then you will have to look at the specifications of new bikes very carefully, since it's usually impractical, if not impossible to address with any given bicycle, although you might lose a couple of millimeters by using a different chainset.