Although there are many things you will learn in your academic career that you may never use again, there are just as many that you will use on a regular basis or will come in handy when you least expect it. The basics of any subject will never let you down.
You DO need to know how to add, subtract, multiply and divide in YOUR HEAD. Why? Because people make mistakes when inputting data into calculators and computers. The answer that comes out of those devices is only as good as the data that went in. If you are shopping and something is advertised as 25% percent off, you should be able to do the mental math to determine if it really is a good deal or not.
When applying for a job and writing a cover letter or resume, you better be using grammatically correct sentence structure or the person who is reading either one will decide that if you can't write a simple cover letter, you aren't worth the time to interview.
Learning about cultures other than your own will be invaluable in the work place when you have to interact with people of other cultures - do you really want to risk offending a new potential customer because you don't understand his cultural background?
I could go on, but you get the idea. In essence - knowledge is never wasted. Embrace the new things you learn and try to see the potential value in each new piece of information.