Good question.

There are at least three reasons:

1) Noto Sans proper has exactly two weights, Regular and Bold, which doesn’t really equal the seven weights of Source Han Sans and Noto Sans CJK.

2) Noto Sans CJK (and thus Source Han Sans) is designed to harmonize with the existing weights of both the Noto Sans and Roboto families. (Your question then might have been about including Roboto given that it has a broader weight range than Noto Sans.)

3) When orchestrated via font fallback whereby Noto Sans proper or Roboto is at the head, the Latin portions of Noto Sans CJK is effectively ignored. Including Source Sans as the Western portion allows the fonts to be used stand-alone.

(I still have a long-term hope that Google will consider using Source Sans in lieu of Noto Sans or Roboto. ☺)

Given the examples you cited, I think that the answer is yes, mainly because of the way in which we defined the scope of Simplified versus Traditional Chinese. Simplified Chinese is based on GB 18030, which effectively means all URO and Extension A ideographs have Simplified Chinese forms that conform to China’s conventions. Traditional Chinese is split into Big Five, which follows the Taiwan MOE glyph standard, along with Hong Kong SCS, which also follows (for better or for worse) the Taiwan MOE glyph standard. This was done mainly for consistency, especially given that Hong Kong SCS is effectively an add-on to Big Five. This is the one area in which Source Han Sans is receiving some criticism, which may be deserved.

The best way to find out whether Source Han Sans fulfills your needs is to actually download and use them.

Sounds goos. I’ll check it out. Thanks!

If you have no use for the glyphs for CJK characters, you are much better served by Source Sans Pro, not Source Han Sans.