Whenever someone requests a 'feature' that I know is a bad thing, I always TRY to get them to think about the consequences -- however, we still gotta eat, so we generally will create the feature and charge them heavily for it.
The first thing I tell them is that if they create an 'evil' toolbar, what will probably happen is that they will:
a) get bad publicity
b) get their toolbar added to the lists of spyware/parasiteware/scumware products
c) get their toolbar added to the removal list of adaware and other spyware removal products
However, in the USA there are a lot of people with the mentality, "we'll do whatever we want NOW, and if we get nailed, we'lll have our lawyers handle it LATER".
Many features of any product or service can be used for good or for evil. A car, for example can be used to drive someone to the hospital -- or to put them there -- depending on how it is used.
A good example is the URL reporting feature of a toolbar. This is where the toolbar reports back to a server whatever URLs the user of the toolbar visits.
The evil part is pretty obvious: spyware
The good part may not be so obvious. For example, the toolbar could customize itself, based upon the web site being viewed, to present the user with different buttons and controls that help them to use/navigate the current site.
A good application of this is one that I'll be launching sometime in the future -- and that is, a single toolbar that gives you 'clones' of toolbars used on the top 1,000 Internet sites. For example, if you navigate to google.com, the toolbar would reconfigure itself to give you a 'clone' of Google's toolbar, then if you navigate to Yahoo!, the toolbar would re-create itself to give you a clone of Yahoo!'s toolbar, etc. (I call this a 'u-bar' or 'universal toolbar' -- instead of installing/removing 1,000 different toolbars -- you would need just ONE!
Anyway, for this particular application, the toolbar DOES report URLs you are visiting back to a server, but the ONLY use made of the information is in order to provide the toolbar user with a new toolbar and better browsing experience. (A GOOD thing!
One of my big peeves is 'drive-by-downloads' -- where someone starts the download/installation process without your knowledge or consent. (You still have to 'accept' the download, but many less knowledgable users may do this without understanding what is happening.)
Fortunately, nobody has asked for that ability (yet...)