I'm not sure about design either. The speed is variable though, so if you were getting it to mix chemicals, you would just use a lower speed. For a yeast starter, you essentially want to get it going as fast as you can without throwing the bar. They also use the exact same teflon coated stir bars. I think the idea behind them is more economical than anything specifically designed for making a yeast starter. Afterall, there are plenty of homebrewers out there using lab stir plates too.
Just thought this might be a good option as they are always available, and you don't have to hunt one down on Evilbay.
I got a Hanna HI 190M from American Science & Surplus in Chicago. It is certainly a hobby version but for about $40 it does the job. I did find, however, that a 1.5" stir bar works more efficiently than the 1" bar that comes with the unit.
Bought a 9 x 9 inch, three torque setting, variable speed magnetic from the evil auction place. Seller was a reconditioned lab equipment house. Very pleased. Made sure I bought a serious sized magnetic cross shape stir bar. If you go industrial size make sure you check the speed setting before doing th Off to ON transition. Mine can spin up water right up the sides. Over kill but works wonderfully.
A stirrer is a nice thing to have. I'm sure tempted to get a higher-capacity one; mine's only good to 1L.
Jim MacKenzie - Regina, Saskatchewan, Canada
A bunch of Nikons; Feds, Zorkis and a Kiev; Pentax 67-II (inherited from my deceased father-in-law); Bronica SQ-A; and a nice Shen Hao 4x5 field camera with 3 decent lenses that needs to be taken outside more. Oh, and as of mid-2012, one of those bodies we don't talk about here.