now here is the question, how long will Chun and Lin be staying in MM? As we all know they are named "transfer student", that means most likely they won't stay in MM for a long time (compare to the amount of time other members have been in MM) and they may not even have an official graduation or something when they do leave MM...
Although Tsunku specially said they are the "key people" for the door to the Asia market (quoted and paraphrased

from reyfer (THANKS BUD), but how I see is this (remember, this is my
assumption!). The amount of time they will stay in MM, as well as this "Asia plan" Tsunku has, will probaby be base on the amount of CD sold for Chun and Lin's debut single with MM...
what I mean is this ---> we know that Tsunku wants to expand H!P to other Asia countries. In order to know whether this will work he needs to know how people (whether they are in Japan or other countries) will react to the group (i.e. MM) with members that are not Japanese in it. To do this, the most basic way is to look at the debut single for Chun and Lin.
If the sell is good/great (better than what currently MM has) then the chance for Chun and Lin (and perhaps the chance of adding new foreign members) will likely be increase, and vice versa if the sell is not good.
so to my conclusion, Tsunku is putting a bet on this. Obviously I hope majority of people will like this new "Asia idea" by Tsunku and that Chun and Lin will stay in MM longer... but in reality it's all depend on the sell and the publicity of them with MM...