The thing is, in a 2000 point army you need 500+ points of core anyway and you'll likely have a castellan BSB as well. It's more of a consentration of points you would already spend.
I know that you have to spend 25% of your army on core one way or another. Maybe I didn't express myself well enough, but my points were that Fireglaives are overpriced, so I would steer away from filling your core with them.
Secondly, a unit costing 700+ points, no matter if you "have to spend them anyway" had better earn it's points back. When playing a 2000 point battle, that's a third of your army in one unit. This means you heavily rely on that unit to perform well if you want to win. A smart opponent will try to prevent that from happening. In my experience, it's not too hard to achieve that when facing a horde of Fireglaives.
Fielding a 700+ point unit turns your army more or less into a "one-trick pony". Fireglaives are not a good trick.
Finally and maybe most importantly, in my opinion fielding such expensive units is simply boring to play with and against
. To each their own, though.