As a new joiner at a company, do you still need to attend orientation training if you've finished your probation period?

According to my experience YES. During your probation period, you do learn issues related to your working area. However in the induction training, you can get information about your company, policies, rules & regulations etc.  

YES ITS A GOOD START AND SIGN OF FINE BEHAVIOUR BY SUBMITTING TO NEW REGULATION WITHOUT QUESTION...SOME PEOPLE FIND IT USELESS CONSIDERING A LONG CAREER AND EXPERIENCE THEY HAD GOTTEN IN PREVIOUS EMPLOYMENT. 

Yes they ought to attend orientation/induction. As already mentioned, this is usually the arena where company information is shared such as corporate structure etc which wouldn't necessarily be included in a local probation plan. It's also important in terms of compliance rates if you are monitored on how many new starters have attended the corporate induction. 

There is a need i guess, because orientation is a function that allows a employee to learn about the organization, what the expectations are in the position, who is responsible and accountable, and in general what they need to know to become an integral part of the company.  Orientation will become handy for you sooner or later. 

from my opinion, yes.

you need or even at least to read their updated SOPs between while and while in order to understand the changes in your company or workplace and to increase your knowledge about the other Departments' work  and standards.