You have Circle of Scorn, Spirits Within, all your multitudes of Tanking Cooldowns, Fight or Flight, and Mercy Blow if Crossclassed.

All of those are Off-GCD abilities you can weave between GCDs to keep things interesting. Making the choice between a Flash for AOE or a Savage/Rage for Single Target is part of the engagement of a paladin. If you didn't have this choice your single target rotation would be identical to your multi target rotation with just a few flashes different.

I can see that Flash off-gcd would make Paladin -better- and -easier- but in no way would it make it more engaging. You're taking out a Choice ("do i use an AOE threat move or a higher threat move on just one target"?) that helps distinguish good from bad tanks.