That is speculation... and bizarre speculation, at that.
There is
zero evidence that powers are in any way balanced with their keywords in mind. Zip. Zilch. Nada.
There are effects commonly tied thematically to damage types. Ditto with tying defense targetted and type of damage.
Cold, Necrotic, Poison? Probably Fortitude quite often. Psychic? Almost always against Will. Fire is often against Reflex.
Want to slow something or immobilize it? Cold is the most common source of that ... fire, not so much. Want to weaken something? Necrotic or Poison is more likely to do that than radiant. Now, blinding or dazing something ... that does fit with radiant a bit more often than cold or acid.
It's not always that way, but if you are a pyromancer, and you have to stick with "natural" fire powers ... you are going to be getting most of your riders being conditional damage (making a zone that deals damage if people enter it) or ongoing damage ... but less dazing, slowing, stunning, forced movement, etc, etc, etc ...
With a feat and a flaming dagger, voila, you get to pick ANY power you want (well, nearly any, it has to deal some damage) and you can make sure you ALWAYS get past resistance, get the best rider effects you can find, and get all the fire boosting riders. And you can grab stuff that targets will and fortitude to be able to have a good mix of defenses to target, instead of taking the "drawback" of specializing by targeting Reflex exclusively, or giving up on fire damage.
The "cost" for a pyromancer to specialize is a feat for arcane implement prof and grabbing a flaming item. Then they can make nearly EVERY power be part of their school.
If it was always intended that implement users can use these kinds of items ... wouldn't it have made sense to have given them implements that do this instead of requiring them to get a weapliment? And of course, screwing over divine, primal and psionic characters (outside of the primals that can use spears as implements) since they don't get arcane implement proficiency to grab a flaming/frost/lightning weapon. [I would love for a weapon that makes the damage thunder, and then be able to increase EVERY damaing burst/blast with the paragon feat]. A lot of stuff, especially stuff from the first book, (remember weapon focus staff for bonus damage ... they FINALLY made implement focus after how long?) seemed to shrug and accept that what they actually wrote didn't do what has intended. They are STILL erratta'ing stuff from the PHB (only with the marshall did they format commander's strike correctly).
Have people been designing powers and class features, and feats, etc ... with the assumption that people can pick up weapliments to just get the energy type they want? Some powers may be that way, but others don't seem that way as much.