Vortecs are a nice pump, that does a very nice job, but yes they are expensive, sure you could get away with multiple smaller & cheaper pumps to do the same job (although you are exaggerating a bit on the price of Koralia & Maxijet mods). However real-estate can become an issue inside of a tank, and sometimes people want a bit of a cleaner look, or something, not to mention the Vortech is the ONLY pump to have the motor not inside the tank, now that might not be an issue/care for some, but for others it is. If you have a prop tank where all you care about is growing corals, then I would absolutely say do a modded maxi-jet or something along those lines, but if you have a nice pretty show tank (or are working for one) having a ton of wires going into the tank to multiple power heads probably isn't what you want either.
There are many ways to do the same thing, and Vortech pumps are by no means the ultimate "must have" for a tank. If you have good flow with other products that are good (i.e. not SEIO pumps
) and are looking for someplace to spend a wad of cash, I probably would rather drop a $850 on a nice skimmer, than a couple vortechs, but then again it seems every few years there's a latest and greatest way to make skimming better changes
End Result: Vortech pumps are by NO MEANS something that people should ever say "You need" on a tank, unfortunately there has been too much of crap like that going along, it was but a few years ago, that if you didn't have a EuroReef skimmer then you had a crap skimmer (at least that's what they told you)... now? EuroReefs are the "crap skimmers"
To follow up on what Paradox said, I've seen the pictures of the uber crazed tank from a vortech pump, problem with that was it was an older pump so probably did run quite hot, and it was at the limits of thickness to use, so the spacer was completely removed and it was pump motor directly on the acrylic. At that level I probably would suggest a super sized Tunze or something though. But what happened to that guys tank was just wrong.