I too am partial towards CEC over other streams.
CEC makes sense economically (students have contributed towards the Canadian economy through tuition fees which are typically 3x as much as a PR or Canadian would pay
and these fees were not subsidised by the local government, through goods and services consumed while living in Canada, and taxes while working in Canada). CEC is also probably prone to abuse though with people coming to Canada to get bogus degrees for the purposes of immigration, but I think there are enough checks and balances in place to limit this. The 15-30 points that are given to students who studied here I think is fair and ensures that candidates still need to get other aspects of the CRS system right to get an ITA.
Besides this, there is also a social or societal aspect (cannot think of a better word - hence I only got a 7 in the IELTS Writing section) .
In my case, for example, I have been in Canada since January, 2009. After all this time, I have integrated fairly well with the society and culture over here. I also have not had a history of criminal activities or shady dealings in what is soon to be almost 8 years. Therefore, from a risk/security standpoint I could be classified as low risk. While I am aware that past behaviour doesn't predict future behaviour 100%, it still makes a good base line. Now you may be thinking that this is where the PCC comes in, but perhaps the PCC could potentially be manipulated in some countries? Anyhow, Canada probably trusts its law and order system more than another country's. In today's day and age where there are constant threats from extremism, I am sure the government prefers people who have a proven track record here in Canada without any extremist leanings. Now, could I be brainwashed into becoming an idiot and doing idiotic things after getting my PR or even citizenship? Absolutely, but the chances are probably lower.
So for these reasons, I like the CEC better. However, I also agree that other streams play a vital role in attracting the best and brightest from other countries so I am definitely not of the opinion that only CEC must exist.
*Full disclosure*: I am CEC so conflict of interest? For what it's worth, I tried my best to be objective and non-partial but that's for you lot to decide.