I have never used a stir plate, so I cannot comment on their effectiveness or detriment, but I can say that they are absolutely not necessary. I have always followed the "shake it whenever you walk by it" method of aerating starters, and it has always worked fine. I did notice a big improvement when I started using the "Shaken, not stirred" method of starter aeration. My starters have taken off more rapidly, smell better (I take this as a subjective sign of yeast health), and have noticeably shorter lag times when pitched.
I'm not saying that everyone needs to toss their stir plate, but it is by no means a required piece of equipment. Your money is much better spent on a temp controller, a new keg, a sack of grain, etc. - unless you're a gadget addict and just want one for the fun of it.