
Christina B. answered 04/02/19
Experienced, Positive, and Effective Tutor with an M.Ed.
It is a very popular strategy and a useful one.
Fluent readers recognize whole words, so we do not sound out each letter or syllable, but emerging readers often labor over each sound. So a young child might see "pig" and have to sound out each letter /p/, /i/, /g/ and then blend them slowly together p-i-g before recognizing the word. But if we can teach the rime "ig," it reduces the amount of thinking and blending a new reader must do each time he sees a word with this rime. Then it is easier to learn big, dig, fig, jig, etc. For the reader it seems like the word contains two sounds instead of three. The burden of learning similar words feels a third easier.
The onset-rime strategy makes the most sense when teaching one-syllable words. In an American school you would see this being used in kindergarten and first grade classrooms. It would come after children are comfortable understanding that each letter corresponds with one sound and before children move on to multi-syllable words. You are correct that "chunks" within multi-syllable words are different. There are better strategies for these situations.