To pose differing beliefs and philosophies to a classroom so that our children may come to conclusions on their own.
To say that 2 + 2 = 4 is not a "philosophy" or a "belief system."
If someone came along and insisted 2 + 2 = 5 without offering any proof or evidence as to why, would you respect their "belief" or write them off as a fool for denying logic and the already reasonable evidence that 2 + 2 is 4 and not 5?
The same case could be made for evolution. The best evidence we have in our possession points to evolution as the reason for our existence. There have been no other solid theories to explain our existence (to my knowledge).
However, now we have people coming along saying, "No evolution is wrong. An invisible man in the sky said a few magic words and created the entire universe in a matter of a week." And the kicker is, they offer no evidence or logical basis for their statement, just like the guy who says 2 + 2 = 5. Are they any different from the guy who thinks 2 + 2 = 5?