Does sending to Kindergarten help getting a place in primary school? In our case I guess it did as the school related to our kindergarten (also private, but very "local") more or less accepted our little one without a lot of fuss while the other school (international) made us go through the complete multiple interviews process and their various presentataions (where they said for every spot there are like 70 something kids applying). So from a feeling, I guess it helps.
We didn't send ours to prenursery, let them be kids... however, she started (a private local Cantonese) Kindergarten with 3+ and she loved it. The kids are really nice in the local kindergartens, in my opinion better behaved and nicer (somebody will come in and shoot me for that) than the brats at international kindergartens... (my observation). She did not speak a lot of canto at that time, but within 6 months she caught up and is fluent (for her age) now. Some of our friends have their kids in english/international kindergartens, but when they play with local kids (ie. soccer class) they simply can't talk or understand the coach...
If you intend to stay in HK for a while, I'd say send your kid(s) to kindergarten and try to find a good balance between local and international, or just local all the way (there is still time for taking it more serious about "learning" at a later age) to get them some canto. The socializing will do the good, and so will speaking Cantonese. If you don't intend to stay, well I'd still send them to some kindergarten for at least 2 years to get them a good social life outside of the home and increase your chances of getting into a primary school.