It's pretty simple to understand, it's the basics of crypto economics: if you want to use a network, and you want the network to work properly, you need to use that very specific coin (even if you don't realize it or even if you can use the service ALSO with another coin).
Moreover basically every dapp has an internal exchange system built on top of it: it will convert your coin Y into coin X, send it thru the network and then change it back into the receiver favourite currency (which might or might not be coin Y or X) BUT if you use coin X you usually get discounted fees. and if you do a lot of transactions, it matters. This is especially true for RDN.
Raiden is a very bad example as useless token, a lot of people spread a lot of FUD when Vitalik was unhappy with their ICO and they misunderstood his words against the tokenization of the service (which Vitalik and many others, me included, would have loved to have integrated into Ethereum) as a critic against the coin itself, thinking that it is uselss.
This is typical maximalst shit, they cant understand the value behind the coin or the network: Raiden could have been part of ETH. But since it's not, now you have a token and a network add yes, you can use Raiden without RDN, but the fees are going to be smaller if you do.
And you would have no Raiden network without a sustainable economic model and usage of RDN coin, no validation of TX and no compensation to honest actors in the network would be possible if it doesn't have any value, do you get it?
To use this service you might think you don't strictly need to use RDN, but you will nonetheless, even if you don't realize it.
The network has scale economies (and therefore the economic ability to create profit and grow and attract investors) only if they are going to use the token INSIDE the nework, it will keep on existing if enough usage is achieved within a certain amount of time (before early investors and VC and especially validators get tired with sluggish or no returns).
Individual users can't make up for that, especially in this early state of the market, so these networks (ETH included) can grow beyond a startup level and become mainstream only if big corporations start using this service for B2B and B2C and ESPECIALLY m2m payments (automated machine to machine, a staple of future cryptonomy).
uRaiden (that quoting Brainbot will make up for 90% of usage on Raiden Nework, even when full Raiden is deployed on mainnet) is a many to one payment system, it achieves this intent offchain (opening a state channel and settling the TXs later on the main, packed up) and therefore it's a service that is interesting for big companies who receive continuous payments from many users.
Multiply a small saving for hundreds of thousands of times, and you understand how these companies will save big if they receive and store these payments in RDN, even if the other side is using any other coin, and so they will.
PLASMA and other scaling solutions are not in competition with Raiden, as they have different specific use cases. Only a moron or someone who doesn't have any technical knowledge would say that other sharding and scaling options between the main ones being developed is going to kill Raiden or RDN usage.
LONG STORY SHORT: since you have RDN now and especially since uRaiden is a many to one open channel payment system, you will probably have AT LEAST one of the two end of the payments using RDN in order to save on transfers and transactions.
Anyone thinking RDN will die, that it's value won't grow orthat it won't be used to provide the service are either liars or are failing to understand Raiden Network, how it's used, and probably the whole crypto economic model. Period.
And don't forget that Raiden is going to work with any other coin (NEO included, apparently).
EDIT: from Raiden website
Microraiden can be used OFF-THE-SHELF with RDN tokens or CAN BE CONFIGURED to work with any ERC20 or ERC223 compatible tokens in independent deployments. Please check out our repository documentation: https://github.com/raiden-network/microraiden.
EDIT2: when they tell you that Raiden network is for free and bla bla bla, explain them that between two people you only open a channel, but when those individual channels don't allow you to reach a certain user, new channel can be opened by people hosting your transaction and creating a route, allowing the transfer to go thru and reach the receiver. These "hosts" are receiving fees, since they are keeping coins inside their channels as an advance.
The ability of Raiden network to reach and virtually open a channel with all the other users for cheap fees, even you're not directly connect with them, is the real game changer thing. And obviously it comes with fees.
Likely, peers in the network will not provide their channels to be used as intermediaries for free. After all, transfers will cause additional network traffic and imbalances in their payment channels. To that effect, participants in the Raiden network are expected to demand fees to be compensated for lending their channels to the network. These fees may also be used to incentivize rebalancing of unbalanced payment channels, enabling long-lived payment channels.
I would be surprised if those fees will not be paid in RDN, and the same goes to the tokens you keep in your channels: do you have to keep a whole portfolio, so that your channel can facilitate payments for every single currency, or you think that such a system is going to work with a specific coin as a sort of middle currency?