Pretty much what it says on the title...
Under the new subscription licensing, viewers can not create new custom view but they can view ones already created (this is fine).
However, of the ones already created by a different user, they currently cant set one of these to be their default (see image below with no "Set as default" option.
This seems like a limitation in the "viewer" user functionality that is somewhat pointless and is creating a couple of extra un-required clicks each time.
I believe the option to allow Viewers to set their custom view should be available.
----> Community Manager Update: This idea was 'Released' with 2019.2 release of Tableau
Special thanks to Andrew Kritzer and team for their hard work on this feature - and thank you to everyone who commented and voted!
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-- Amanda Boyle, Tableau Community Manager