It is very easy to just click "ok" when a dialog box pops up. How many times have you ever actually read the terms of service or end user agreements as opposed to just clicking yes? How often have you been on a website you visit frequently and just click yes or ok without actually reading what the box is saying?
I've seen it happen to colleagues and friends a few times where they think they are on one site and don't realize that the pop up box is actually for malware or bloatware. Of course, that quick click takes much longer to undo after all sorts of toolbars are loaded.
Sometimes it is hard to tell whether an add-on is actually something useful. Here, I take the approach of running a quick search on Google for the name of the add-on. If the first 5 results all are titled something to the effect of "How to remove [add-on] toolbar from you system", run don't walk away from that box. A 30 second Google search can easily save a much greater amount of clean up time if you download something you really don't want on your computer.