The most important use of folic acid, is the prevention of neural tube defects (such as spina bifida).
These are formed by 12 weeks, which is why folic acid is so important in early pregnancy, and indeed when trying to conceive.
With DD1 I was taking Epilim (sodium valproate), which is one of the most dangerous drugs to take in early pregnancy, highly teratogenic. So much so, that some doctors advocate abortion.
I was stopped immediately, and placed on 5mg of folic acid for three months (considering how much (400mcg) is in prenatal supplements, you can see I was on a mega dose.
I was told it was not necessary after three months, as the neural tubes were formed, and as I had more reason to take it than an average pregnant woman, you'd think they'd have continued if it were in anyway helpful.
That said I agree with you, as I took ordinary pre-natal tablets, I still developed anaemia though 