Is it necessary for a company to be present on all social media platforms?

It depends on what the company or brand can get out of social media, and most importantly to pick the right platform for their businesses.
A company usually asks itself the 2 main golden questions: "Why do I want to pursue social media?" and "Where are my target audience?" If those 2 questions were properly answered and the business objectives are clear, then the answer to this question would be even clearer.
Picking the right platform is what brands and companies should be concentrating on.
For example, Why would a painting artist be on Twitter? Or why would a Radio company be on Pinterest? You see, since "Content is KING" I would personally argue and say it all depends on "how" you can provide your content on those channels and how you can engage with your audiences to drive traffic.

Yes.
Simply for the reason that someone else doesn't come in and squat on your profile.
i.e.
maybe you don't update your Google+ everyday but at least claim the vanity URL.
As far as an active presence goes, only do what is relevant to you while being unique.
If your Twitter, Facebook, Google+ all look the same as your website, it is pointless.
Evaluate the strengths of each social network and leverage that to your benefit.
If you can't have an active presence all the time, at minimum keep all your links updated and point any interested parties to your most active social network.
Several companies make the mistake of using platforms just to speak, rather social is about speaking AND listening.
My analogy would be, don't go to a party if you're just planning to sit in the corner and listen to your iPod.

Of course not.
Being present on every social media platform is not a requirement for the success of your social media strategy, and can often prove to be a waste of time, effort and money.
What I recommend social media managers to do is to conduct a test-drive for all the social media platforms they are interested in, and then see where their efforts are most fruitful.
Based on the results of the test-drives, social media managers could then choose which social media platforms to keep and build on, and which social media platforms to discard.

I would say it's necessary to be on all high-profile social media platforms.
Bear minimum, make sure your profile is on Facebook, LinkedIn, and Google+.
As a company, you would always want to increase the number of opportunities to be discovered, whether it's through social media sites directly, or through social media sites via their rankings on Google, Yahoo, and other search engines.
Remember, you would want to claim and own your company profile on social media sites before someone else does.
Make your profile an important extension of your company's online presence.

Yes, In order to attract more people and different audience.
Also it can help you increase visibility of your company.

I think it is good for a company to be present on all social media platforms, if not for marketing reasons and reaching out, then at least to ensure that users can find that presence when they search and to avoid squatters.

In my humble opinion no, I believe having focus is important for success. A company needs to be present in social media where their consumer market is present