Yes, you will get an additional profit when trading the ETH/BTC pair if the market is in a bullish phase. But if you continue to trade this pair when the market changes to bearish, then your profit will be significantly reduced, and perhaps even reduced in dollar equivalent.
Of course, this applies to the negative scenario just as it does to the positive scenario. Although I only mentioned the positive scenario in my comment because the content is based on a positive scenario, a similar situation will also apply to the negative scenario. For this reason, it is always beneficial to choose the transaction parity to be preferred by taking into consideration the market conditions. In addition, it shouldn't be forgotten that in the event of a possible negative scenario if Bitcoin dominance becomes high and Bitcoin loses value there will be a loss both on Bitcoin basis and USD basis due to parity (like a double loss). Certainly, choosing transaction parity considering both scenarios will minimize possible risks.
I feel like the near future looks quite bullish, this is why we tend to look for the situation to be a bit more positive, and that is why it is normal to think about the bullish period. Plus, OP mentions "profitable" which means that it makes people think about the bullish period a lot more. I think it should be noted that we are talking about a situation that is not going to be all that simple, because usd and eth are good for very different periods. I am not saying that it is not going to end up with a bad situation, there could be moments when it is bad, or I could be totally wrong and it doesn't go well at all.
However, if you look at the market right now, and check out the few months we had so far, I would be easily saying that it has been great and will continue to do that. In that case, if the price keeps being bullish like it has been so far, then I am going to guess that it is going to be as great as it gets and you will be right to pick that pair.