At first the categories are organized as following:
"•Category 1: For persons in an occupation which corresponds to the list of occupations identified in the Ministerial Instructions and have evidence of one year of continuous full-time (or full-time equivalent) experience in that occupation;
•Category 2: For persons who have arranged employment offer with a Canadian employer; or
•Category 3: For students or workers who are legally residing in Canada, and have done so for at least 12 months immediately before submitting their application."
Have a look at the link :
http://www.cic.gc.ca/english/information/applications/skilled.asp
secondly:
As i told you each one of the three categories is seperate from the others. they don't consider or give any one of them more priority than the other but people have different situations, that is why they give three options for the FSW class.
Going back to your point; the difference between category 2 and three (arranged employment) and (people who have been working or studying in canada for at least one year) is in the requirement documents that you have to submit and your current situation.
some foreign workers who have been living in canada for more than one year with positive(LMO), cannot get a permanent job offer from their employers, that is why CIC made a seperate category for them which is category (3), so that, in order for them to be qualified, they have to send documents prove that they have been working legally in canada for at least one year,and in this case no (Arranged employment opinion)(AEO) is required, just a copy of their work permit and letter from their employers confirmS their period of work at their companies.
some other people who have been legally working in canada for one year or more are LMO exempted such as (post graduate work permit, dependents work permit,...and so on. Therefore, they can apply under category (3) without both, (LMO) or (AEO).
Category 3 was made to serve people who are already working or studying in canada legally for at least one year and not for people who live outside canada and thinking to come to canada to stay one year and then apply under FSW.
you need to understand that LMO- (labour market opinion) is different than AEO- (arranged employment opinion),
LMO is for people who wnat to come to canada as a temporary foreign workers and not as an immigrants, or for people who is currently working in canada and need to renew their work permits.
Arranged employment opinion is for people who intend to immigrate to canada and does not support getting a work permit at all.
If someone is working legally in canada for less than one year but he has a permanent job offer, he can apply right away to immigrate under category (2),
If someone is working legally in canada for one year or more and he doesn't have a permanent job offer, he can apply to immigrate under category (3)
If someone is working legally in canada for one year or more and he has a job offer as well, he can apply under any one of the two categories (2) or (3) it is up to him.
If someone has been working or studying in canada for less than one year and he doesn't have a permanent job offer, or his job title doesn't fall under one of the 38 list of jobs, he will not be eligible to apply under the FSW category 3 (which is his only choice) before reaching one year in canada.
Getting work or study permit visas alone does not give any one the eligibility right away to apply under category 3, otherwise, all people around the world would apply to get those temporary visas and apply right away under category 3 becuase they have just got the status as a temporary residents of canada and then, leave canada for example after 1 or two months !!
That is why CIC made it one year, just to make sure that the person (who doesn't fall under the two other categories 1 and 2) was a bona fide student or foreign worker at that period of time and not only because he has got the temporary resident status recently.
