SIG did so not to reduce milling in the 220, true, but it is more of a product of the slide construction. The slide was stamped (though folded is the current description), and it is just plain impractical to round a slide and mill lugs when your goal is to stamp its shape. The question is did the single large lug lead to the folded slide or the folded slide necessitated the use of a square lug? Either way, Glock copied it 7 years after SIG introduced it and so it is hardly original on the Glock - indeed nothing is original on the Glock - all features were pioneered by other companies in other pistols. Of course, Glock combined the features into a single pistol, and so the Glock was the first, you could argue, to do it all at the same time. It is, in short, an example of the best that Browning, SIG, Hk, Lee, and others created over more than 100 years.
The squared-slide profile is popular, but there are many designs that do not use it. They are all solid, rugged, and reliable.