For today the cource of an etherium makes about $860 it means that one OAK token is hardly more dearly than $1. Now it is still profitable to invest even at this stage. Because the declared price of the OAK token is $ 1.4
And don't forget Acorn still doesn't announced main sale bonuses. May be it will be tasty. 
If price will be ~$0,9 (1ETH -> 800 OAK) it's like a discount already

Pre sale was for ~$0,7 and it suppose to be 50% discount in comparison to main ICO for taking risk (no refund, still no softcap hit etc.).
So i wouldn't expect too much discount, we'll see.
Quote from: WhitePaper
Exchange Rate
1 OAK = USD 0.70 (Pre-Sale price)
1 OAK = USD 1.40 (Main ICO price)
Final OAK vs ETH exchange rate will
be fixed on ICO begin, to protect intended
USD raise target.
Pre-Sale 7,000,000 OAK (50% discount)
1 OAK = USD 0.70 (Pre-Sale price)
1 OAK = USD 1.40 (Main ICO price)
Final OAK vs ETH exchange rate will
be fixed on ICO begin, to protect intended
USD raise target.
Pre-Sale 7,000,000 OAK (50% discount)
I think he did not expect to give such a big discount at the main stage of sales, the ethereum tokens because fell in price.
Well, keeping the price on a certain level isn't the goal of ICO, right ?

I'm sharing your concerns about ETH price fluctuation, in the worst case scenario it can kill main sale. I'm really curious about this main sale structure.