I agree. Bitcoins success as a currency lies in people buying into the concept. Sure network security is a significant factor to consider, but it's everyday usefulness, as well as price stability is a much more significant one. Just look at countries around the world where the USD is/has been the defacto currency due to the native currency's hyperinflation. Bitcoin will suffer the same fate if at some point the price doesn't settle more.
This is a really interesting point. Part of the developing infrastructure that is fascinating, however, is increasing the ease of converting fiat to BTC and back. Allowing quicker and easier conversion could allow Bitcoin or a different cryptocurrency with faster transaction times for smaller purchases to be used while not having to be affected much at all by the volatility, in lieu of services like Paypal or in situations or areas where credit isn't as easily accessible.
It's possible that ASICs will push the price down too I think. The more BTC controlled by a very small number of parties (AM for one, Avalon perhaps another), the less likely new people will buy in. Why? For the same reason BTC became popular in the first place, decentralization (or in this case a move towards centralization as the ASIC companies gain a larger and larger percentage of the currency as a whole.) You could probably argue that these companies will not risk price manipulation in order to maximize long term profits etc, but they are in this for the money too, esp AM as they've proved with their overpriced USBs. With risk of price manipulation, the attractiveness will drop.
This is where things could really get shaken up over the next year or so. ASICs are currently available to people who have a significant amount of Bitcoin already. They are also application specific so it's equipment with little to no resale value it Bitcoin goes away. I think either that reason or yours above of potential centralization could combine to make an altcoin like LTC relatively more attractive in the short term until ASICs can be bought with fiat and on a smaller scale. That said, I do think companies like AM and Avalon and the first-movers who invest there deserve to make a good to great ROI.
I also think the USBs are also overpriced, but they have a monopoly right now - and people are showing they are willing to pay the price. Hopefully the subsequent rounds of them are priced lower and they can act like the entry-level GPUs of the energy efficient realm.