They have already successfully given powers to Nikki, Tracy, and Nathan as far as we know, and possibly others

So why would they need Mohinder to "perfect" the formula?

I don't think they actually "need" Mohinder, I think it was just a way to get him to join up with Pinehearst. Arthur seems to really have planned everything out down to the last detail, hence all the files and assignments, so I'm assuming Mohinder has a part to play in Aurthur's grand scheme. What it is and what he has planned for everyone else in those files is yet to be seen.

Mohinder has the smarts to figure it out. I think it's also that they need Mohinder on their team, instead of the opposition.

That too. But sometimes to make something work you need two camps. I think the formula they have changes your genes, but look how long they had to wait for Nathan and Tracy to manifest. I think Arthur wants to make HIS formula for right away - hence Mo.

The tv wasn't invented until a rivial project saw what the competition was doing and combined it with their own.

I don't think it was Pinehearst that gave all the babies their powers, but rather the Company with all the original founding members. Remember Angela's statement when she was telling them that they "Thought they were gods"?
Then there seems to be a rift [or two or...] where Adam and his lackey Linderman wanted to destroy the world or most of it and the founding members didn't, then Arthur wanted somehting else [Magneto and his Band of Mutanats] so that they could rule over lesser humans, etc. so we will see where all this leads, whether Arthur was the original perp and Adam/Linderman were his cronies and defected [i.e. his revenge by taking Adam's power] or if there were two different factions in the Company.
Regardless it seems that Arthur wants to control all Supers' and need SpiderMo on his side rather than having him on the other side.

Pinehearst didn't give powers to Nathan, Tracy/Niki/Barbara - that was The Company that Angela and the twelve founders created. The two pieces of the formula were being held by Angela and Kaito.

There is a theory I placed at the speculation section.

"How old is the formula?"

Kaito mentions it is a secret kept in the family from his father.

Angela said Zimmerman "helped" develop the formula to give abilities.

Most telling is that Adam told the founders to destroy it.

Which could mean the formula is just a template for a successful formula.

There are some details that is missing that should be filled up.

The formula is older than we thought. Perhaps streching back to WW2.

Or it's actual very new.

Maybe Mohinder first developed the formula, and time travelling ensures it gets back to the Company and Pinehearst?

The opportunities for shattering the space/time continuum are plentiful in that scenario!

That's kind of what I thought was going on, Mohinder created it now or in the near future and it was taken by the company through time travel and that's how Zimmerman got involved.

When they were showing Mohinder the formula, didn't it look like the left side of the formula was missing?

I thought they had said that it was in two pieces, but it definately looked like the left side wasn't missing - maybe there is a third piece that was lost/destroyed and they need Mohinder to recreate that part of it.

-p

The other thing that they made need Mohinder for is finding a way to take away powers without Mr. Petrelli doing it. There maybe powers that he doesn't want, that are too hard to control, but still doesn't want others to have. Like Ted for example.