The Yahoo tool bar is an add-on that usually gets installed accidentally for most people when they install other software that it "tags along with".
There are lots of tool bars, the Bing bar and the Google tool bar and on and on. most of them are NOT made by Microsoft.
Normally if a tool bar was installed without you wanting it too, then I'd say definitely disable it. But if you find that you like the functionality and it's working well for how you want to search the net, then by all means leave it. I do not believe it's any better or worse than any other add on tool bar out there. It's main bit of "evil" (like all the others) is that it automatically drives all your traffic to Yahoo for searching. But that's not necessarily bad or anything really... as long as the bar was installed because you wanted it there and want to use it
If you don't want it there then by all means, disable it from IE or uninstall it completely... (Click the IE GEar in the upper right and then select "Manage addons")