If the UN has to prevent any wars, it is the Security Council that has to say so, and guess whose voice is loudest there! So if the UN appears useless, you need to direct that question to the United States - it is their fault.
And it is to reduce such US dominance that India shouldn't leave the UN - we should try our damnedest to get into the Security Council, with or without the veto. Once in, the efforts of all nations should be directed towards abolishing the veto.
As far as Kofi is concerned, he was just being practical and telling us the facts - there is nothing he can do either to give us the veto, or to take it away from the P5. It is silly to accuse him of apartheid, when the culprits are the US and its cronies.
It doesn't make sense to equate signing the NPT with UN membership - NPT is only one of many agreements. Even the US hasn't signed so many such agreements. If India leaves the UN, no one will miss us. But we will lose our voice in the international community, and be the only such nation- Switzerland joined up a couple of years back.
Looks like you need to have an in-depth conversation with your friend Shashi Tharoor to get your facts in perspective.