It's quite plausible for a dilithium-rich world to require a subspace field for the trigger or catalyst, considering what we know of this magical substance. Few natural phenomena would exist that could create such a field. So, warp ramming makes for a plausible tactic in this special case.
The storyline from The Great Starship Race
is slightly more difficult to swallow, as the ship doing the ramming is carrying at most a few tons of the substance, unlikely to make a difference even if it were pure antimatter (the Dreadnought missile from the VOY episode indeed had a ton of that, and wasn't supposed to make entire planets disappear). But the supposed result isn't Alderaan-type disintegration anyway: instead, the attack is going to result in an explosion a few miles below the crust that will open a super-volcano of sorts, spewing hot cobalt vapor to burn and poison the surface of the world. A bit less ambitious, a tad more realistic for a WMD based on warp ramming.
Except that warp isn't actually needed in this case to create the destruction. Warp is only needed to deliver the "warhead" through possible defenses, whereas a high sublight speed would supposedly already cater for the called-for penetration of crust. So, a bit of additional realism there, for something this fantastic anyway.