In terms of brand loyalty Google has removed any reason to be "loyal" to their search brand and has been operating on being better then everyone else, however its getting close for most mainstream searches and if I can support a healthy marketplace then I am all for it.
It's true that Yahoo's results are sometimes noticeably worse than Google's. The result I want can often be at the third or fourth position on the page, rather than first (where it most often is on Google). Given that Google is obviously personalizing my results based on my query history (unless the Rust programming language is really the world's most popular interpretation of the term "Rust"...), I'll be curious to see if Yahoo does the same over time.
Plus, search quality is a very subjective matter. I have Bing on my phone for a while (mostly to avoid Google's redirect links), and I hardly notice it any more.
;)