That depends what you mean by "necessary". Many people on this thread are expressing the opinion that at a large number of events amplification is not only not necessary, but it is actually a nuisance.
I was immensely gratified to get an email the other day asking the band I play with to perform at the East Anglian Traditional Music Association annual shindig; and the letter pointed out that all venues at this festival( I think) are acoustic(in the old sense of the word). Great, I thought. Someone's bucking the trend.