It could prove useful even in the era of sail. Hawaii could serve as a sort of stock exchange between the two continents. Traders drop off their goods there and sell them without having to make a cross ocean journey.
I'm not sure it would be of use under sail though. You can't sail straight there from East Asia - you have to go north, follow the current past Alaska, the Pacific Northwest, and California, then cut southwest to the Equatorial current, then pull yourself off that to reach Hawai'i. In other words, you have to sail the whole way across then come halfway back. Mind-boggling waste of time.
The folks coming west from Mexico (or wherever) are in an almost equally weak position. They can get to Hawai'i much more easily, but can't sail back east. They have to sail most of the way to Asia to catch a proper breeze east.
In short, Hawai'i is only in the middle of the Pacific if you have motor transport. If you rely on wind power and want to do a round trip, you have to sail just as far as if you were sailing end-to-end.