Yes, you're right. If it's a free asset pack provided by RS.com, then I would think it would make more sense to just automatically include it with an update to RailWorks to help ensure that all end-users will have it (at least those who keep their RailWorks updated). That way, as route-developers, we could help eliminate the extra step and confusion of directing people to first download this pack before installing any route that would make use of any of these assets.
Hopefully some forthcoming update will include this pack to become part of the RailWorks default-assets.
You can see some exchange of messages I've had with RSDerek over on the UKTrainSim forum in regards to this pack:
http://forums.uktrainsim.com/viewtopic. ... a&start=15 In which one of his replies says: "I think the pack can be considered default, but users will have to go to steam shop to get it for free, in any route released make sure you put that in the read me."
So, for now at least, it's best to be sure to include to tell users to install this pack in the ReadMe file that comes with any routes that may use it.