Of course it has to be added that you need to floss at the same time, but your tooth brush should still be the main mechanism for preventing decay and if it is unable to get into those corners, then it does not matter how much money you spent on it as it means it is not doing its job correctly.
It is just so much more hygienic.
Overall, the main reason why you need to change your tooth brush so often is because of one over-riding reason, it is just much more hygienic. Your mouth is actually quite a delicately balanced ecosystem all on its own and it does not take much for that to be thrown out of balance. Just as you need to be careful about what you eat or if your hands are clean and hygienic, then the same can be said about your tooth brush. However, the problem is that it is difficult to know if your tooth brush is as clean as it could be even if you take all of the relevant precautions that you could read about online. Changing it on a regular basis removes those concerns in an instant, which is why dentist around the world recommend it and why it is seen as being absolutely basic advice.
As you can see, there is a real need to change that tooth brush every 3 months or you could be making life harder for yourself regarding your general health. When you think about it, this is not exactly something that is difficult to do and a brand new tooth brush is not exactly that expensive, so is it worth taking the risk?