I hold that it's ethical so long as you actually have the desire to make good on a future purchase of the series. In fact, there was a point - back 10 years ago - when fansub popularity was an influence on licensing companies choosing which stuff to bring over. One of the higher-ups at ADV admitted - probably during a panel at a convention - that this was the reason they licensed Azumanga Daioh. It's obviously harder to make this 'licensed due to fansub popularity' argument today, since a fairly good portion of titles get licensed and simulcasted on both sides of the Pacific at the same time, but I wouldn't be surprised if there's still cases of it happening with sleeper hits.
Most of the other reasons are related to the technical aspects involved. My computer isn't powerful enough to watch official streams, the subtitle styling they use makes my eyes want to bleed, and I prefer things like original name order and honorifics to be kept (usually; there are exceptions, but not many of them). For those reasons, it's less irritating and more palatable to go with fansubs for the initial watching. Those outside of the USA also have the issue that many of the companies that license series only do so for North America and get blocked from watching even the official streams - short of using proxies anyway. Also that some markets outside of North America obtain demonstrably less titles because anime simply isn't as popular. The biggest three markets outside Japan that I'm aware of are North America (which usually means just the US and Canada), Oceania, and then probably France or the UK or Germany, but it's hard to tell by that point. That leaves a lot of areas with much less (or almost nothing) to pick from legitimately.
I've gotten to the point that I really only watch series that I know I'll end up wanting to buy, so my intention in the end is always to obtain a physical copy when I'm able. Some of the prices make me cry (Aniplex of America selling Bakemonogatari and Nisemonogatari for a combined $290 after sales taxes, which works out to ~$11 an episode; granted, Aniplex is the exception here, and most domestic companies' DVD or Blu-ray sets are far far below the series' original Japanese retail price point), but income-willing, I'd buy them all the same.
