The other currencies you listed (Pecunix, LR) are backed by gold and silver via the USD or EUR, so they are not really new currencies as much as more anonymous forms of paypal. Other e-currencies I have run into over the past couple years are also usually backed by gold or silver ultimately.

Bitcoin, in contrast, does not derive its value from gold and silver. Instead, bitcoin derives its value from the same things that give gold and silver its value. It:
1. is suitable for transactions
2. has limited quantity (scarce)
3. is not arbitrary: it is the currency that has the most widely recognized transaction history, measured by CPU time

I can imagine there being 2-3 different CPU-based currencies like bitcoin with exchange rates as you said, but it would be better for only one such currency to exist to preserve the non-arbitrary nature of the leading currency and promote its legitimacy, which everyone using bitcoinlike currencies will want. I would expect a self-perpetuating tendency toward only one CPU-based currency.

I question whether multiple currencies could co-exist. Nobody wants to receive payment in a currency that has less CPU hours of proof-of-work. If the cryptographic math behind bitcoin makes sense, then people will want to be paid on the proof-of-work that represents the most widely-trusted transaction history, which will usually equate to the proof-of-work with the most blocks / CPU time.

Regarding multiple clients, this is inevitable. With the math behind Bitcoin being open, anyone can write a client. People can and will use the various available clients as long as they can trust the code and the client is following the rules to the satisfaction of the other clients that are building the longest proof-of-work chain at that time. Anyone holding bitcoins will want to support the bitcoin work with their CPU so their wealth stays legitimate. Others will only want to donate their CPU to a network that will acknowledge their work with payments in a legitimate currency.

Agree the network protocol can be changed without branching the proof of work chain. But the rules on what transactions to accept, e.g., the 50 bitcoins going to the author of each block, must stay the same or it will need to be branched. For example if a new client comes out that drops the 50 bitcoins into my account instead of the authors account will work among the new clients but existing clients will not build on top of those invalid transactions.

It would be necessary to make random periodic exchanges with other bitcoin holders to separate your identity from your bitcoins.

Any new client that decides to change the rules would have to branch the chain.

If the branched chain can not keep up with the original chain then individuals and businesses will eventually reject transactions made on that chain, and honor only transactions on the larger original chain.

If Bitcoin decides to make an unpopular rule in its next version release, it too would require a branch (because older versions are not familiar with the new rules) then concerned individuals will encourage others to roll back to the earlier version and the earlier version's chain would remain longer than the new version and it would be the only accepted history of transactions.

So I do not think anything bad could come of allowing other clients to exist on the network. In fact it is the only way for the currency to survive. People will not trust an arbitrary closed standard.