Later in the series, a mysterious villain named Vadic (Amanda Plummer) who pilots a talon-like ship called the Shrike will reveal that she has been controlling the portal weapon, and will happily demonstrate how functional it can serve in ship-to-ship combat. When her prey attempts to flee, for instance, she can project a portal in front of it, redirecting them directly in front of her ship again. If she’s fast enough on the draw, she can open a portal in front of a photon torpedo, forcing it directly back at her attacker. The uses of the portal weapon are limited, it seems, only by the imagination and ruthlessness of its wielder. 

To idly postulate, one could open a portal right next to a sun’s cornea, position its exit next to a ship, or even a planet, and blast either with unbelievable solar energy. One could open a portal in front of a ship, and arrange its exit immediately behind, allowing a ship to rear-end itself. One could fire a ship’s phasers or photon torpedoes into a portal, and arrange its exit inside a ship, or even inside a deep basement somewhere, essentially committing acts of terrorism from space. The building-dropping maneuver is plenty creatively destructive as is. 

Because of these weaponized uses, it’s clear why portal technology has been banned and hidden away from Starfleet at large. Yes, the portal tech could make rescue and transportation that much swifter, perhaps being used to evacuate thousands of people at once from dangerous situations. It would also give researchers a chance to, say, scan dangerous spatial phenomena from a safe distance. Just open a portal and scan away. But until a portal cannot be used as a weapon, then perhaps wait to use it.