> Why do people keep trying to argue that dogecoin/litecoin/etc/etc don't have value, when people trade them for other currencies every day on open markets?

They have value but it's much more based on speculation. If you look at Bitcoin, there's an ecosystem of millions of users, over 100 thousand merchants including some of the world's biggest companies like Microsoft, hundreds of bitcoin-focused VC-backed startups of which one had a valuation nearing half a billion dollars etc etc.

While it's obvious that bitcoins are overvalued in relation to this ecosystem (i.e. a substantial part of its price is based on speculation that the ecosystem will become massively important in the future), there already is a genuine ecosystem out there with decent growth rates.

Dogecoin or litecoin don't have that. They have communities that keep them alive but there doesn't seem to be a long term viability. This is different from bitcoin's early days (which didn't have any real ecosystem either at one point) in that bitcoin actually offered something radically new, while litecoin for example isn't innovative, it was the first me-too that didn't totally screw over its early investors in a pump & dump scheme so it's stayed around but it doesn't offer anything special. Does that mean they have no value? Absolutely not. They do. But 'me-too' companies also have value in the short-term. In the long term they usually don't survive. We're already seeing that, litecoin for example dropped insanely in price (like 90%) in the past year and that's not just because bitcoin readjusted after late 2013's hype cycle, it crashed against bitcoin itself, too, i.e. the market is already identifying that litecoin versus just bitcoin doesn't have a future in the long-run. And if there's ANYTHING you have to know about bitcoin is that it's almost always spoken about in a long-term frame of reference. If we purely look at the short term then yes, we can say dogecoin & litecoin have value and so does bitcoin, but we would also have to say that all three combined are absolutely insignificant, a drop in the ocean, the size of a mid-cap fund of which there are thousands. The short-term isn't interesting, in the short-term me-toos exist, in the long-run they don't.

In short, the reason people say litecoin has no value is not because it's not being traded: that's speculation. It's that it is said to have no utility value over bitcoin. Bitcoin can power any utility litecoin can, with more liquidity, more market participants, more open-source libraries & development, more funding, more security etc.

I don't think we'll all be using bitcoins necessarily, but I do believe we'll all be using bitcoin (or no cryptocurrency at all, of course, that's very possible, too). i.e. yes there are niches in digital currency, but the cryptocurrency niches will, I think, if they exist in the long-term, be served by products running on top of the bitcoin network. Anything litecoin or dogecoin can do, a bitcoin treechain or sidechain or payment channel can do but with the above mentioned benefits. It's like saying there are different niches in internet protocols, as if to say the internet may run on many different protocols that serve different niches in the long-term, instead of saying TCP/IP powers the internet and it has higher-level protocols that serve niches like http or smtp. Within that line of thinking, anything but TCP/IP has no long-term value and any niche in internet technology will be built on top of that one protocol. Similarly many who say 'litecoin has no value' think of bitcoin as being the protocol for value transfer, and anything having to do with transferring value (or titles, contracts etc) will be built on top of it, rather than along side it.

In short, the claim that litecoin has no value (despite having a market value) makes a lot of sense depending on what you believe about the future of cryptocurrency.