Do you think own a toolbar on website is helpful to get returning visitors? Would you like to own a piece of toolbar or does not care?
i think unless it's a trusted site, most people will not download a tool-bar for fear of spyware. And besides after you get yahoo and/or google toolbar, another one needs to be pretty special...what will it offer that is unique...answer those things and you will be successful. Good luck!
The only way I would download another toolbar is if it offered a service that benefited me. I downloaded the StumbleUpon toolbar because it allows me to surf and book websites really easily. I also have the Alexa/Pagerank toolbar. Other than that I use FF with the plugins. So unless your site has a cool service that people are interested in using for continuously, I'm not sure that you would be able to get them to use your toolbar. Good luck with your venture though.