3 months notice period-Totally ridiculous or an absolute requirement?

Taking off from a comment in my last post I am increasingly curious about the merits of having a notice period, especially the 3 month notice periods

The main reasons why Organizations have a notice period are:

Backfill

Since finding the right profile may take time it gives an opportunity for the Organization to backfill the position. However, this challenge is more in smaller Organizations when there is only one person doing a role. In larger Organizations, there should be others who can fill the gap temporarily.

Knowledge Transfer

It takes time for new hires to come up the learning curve and having a tenured employee around will help in the learning process. The employee leaving should pretty much buddy up with his/her replacement making them feel comfortable during their early weeks

Maintain continuity

It gives clients a sense of comfort knowing that the movement is seamless and there will be little or no impact on customer satisfaction.

Retention

Organizations should step up their efforts to retain the employee. The notice period is a great time to engage more with the employee and explore opportunities for retention.

What started off as a 1 month notice period has somewhere along the way become a 3 month notice period in most Organisations.

While I appreciate that it is difficult to rehire and recruitment teams are under tremendous pressure I can’t see the advantages of having someone continue to work 3 months from the time s/he have submitted their resignation’s.

A 3 month notice period seems month notice period in most Organisations.

Disadvantages of having a 3 month notice period

Billing

I have heard that one of the reasons organisations have a 3 month notice is also because they can bill the employee during this time. Makes sense but it may end up with clients being unhappy with poor performance. Technically, what we are doing is having an employee who is performing below par but billing the client at 100%. Not fair, in my opinion. Perhaps, debatable.

Disengagement

Employees on notice period would normally be disengaged. Disengaged employees perform below par and this could have serious impact on performance and client satisfaction.

Disgruntlement

Unhappy employees tend to bad mouth organisations and this can have an negative impact on employees working. Imagine having a colleague crib and curse continuously for 3 months!
Doesn’t it make sense for all Organisations to move back to a 1 month notice? Please share your views.