The fact that it pays in BTC is a good reason. Right now BTC is the safer bet in a very high risk market.

If you mine to hold any other altcoin you will need more luck than doing this way with nicehash, as you can check by yourself, mostly of coins are droping in value when compared to BTC price (parameter that people tends to ignore). If you can have either BTC or ETH for example and choose the later, even if ETH rises 10% it's no indicator to a best profit, you need to check if ETH increased or droped value when compared to BTC, if it droped, even a bit, it means that BTC was a better option.

Since this is the case nowadays for the majority of grand coins, it means that BTC is not only a safer bet for being more widespread, but it's a safer bet for having a tendency to hold more value when compared to other coins.

I understand that you can have more profit if you "sniffdog" coins yourself, but not everybody can live it like this or have enough time to do so.