Though I bought most of my ETH for ICO, the first time when I bought it, I wasn't aware it is being used as payment for ICOs.
I bought ethereum when it was only worth $16 and icos weren't really around (or at least weren't so popular). I sold ethereum when the ico craziness was around and am rather happy because of that. The coin is quite good but security problems are not so nice. I hope we'll see some improvement in a week after the switch to pos we've been waiting for. Yet I think that as for an approximate amount of people buying eth solely for icos - it must be about 80% of all the purposes, because the price growth is mainly based on icos being built.