The only effect of adding coins from a network that is near duplicate of Bitcoin (and Litecoin is a near duplicate of Bitcoin) ...
Litecoin may be similar to Bitcoin, but it's not Bitcoin. That's actually the reason
I began promoting its adoption. Even if there was another coin called Bitcoin with the exact same settings, but with a different imprint of coin owners and development team it wouldn't be Bitcoin (the one we call Bitcoin on MtGox).
In fact Litecoin does have other key differences. One is its hashing algorithm, which provides an in place backup if Bitcoin's ever breaks (or vice versa). Another is its blocks being 4x faster. Anyone on this forum a significant length of time will recall multiple threads complaining about not receiving even 1 confirmation, even when paying fees, sometimes for hours. Time is a factor in payments. To think it's not I think is frankly unrealistic.
... to an exchange is increasing the supply of coins.
Let's put things in perspective shall we? Bitcoin and other cryptocurrencies thus far limit their total supply to some number of millions, Bitcoin's being 21 million and Litecoin 84 million, for example. There is a world population exceeding
7 billion and growing. Meanwhile, there are dollars (alone) with monetary unit supply in the several
trillions. Does that help?
Instead of diluting the Bitcoin market with a copycat blockchain ...
The point of being a different coin is that it's
not the same blockchain. Also, since when is there a "Bitcoin" market? Isn't there only a free global market?
... with 84 million additional coins, MtGox should redenominate BTC to mBTC so that people looking for 'cheap' coins can get them.
That doesn't mean cheap coins. Google's stock price is currently $880 per share. If I say I'll sell you .001 of a Google share for .88 cents each do you really think you've gotten "cheap" Google stock?