Advantages: Quick to make
Humanity has just invented the means to travel out of the galaxy, into 'deeper space' meaning that the aliens are now going to attack.
They have developed electrical equipment, which has opened the way to advanced weaponry.
When the aliens left, humanity stopped fighting against them, and started using the altars against each other. This led to disappointment on behalf of the 'nicer' people, who shunned the practice of summoning, and labelled it 'evil.'
Eventually, summoning was forgotten, and all that remained was the legends of demon-summoning.
In the year 2105, humanity developed space flight advanced enough to take them out of the galaxy. They were fought back by the aliens, who pushed them back to earth.
The humans led the aliens to Australia. Here, the aliens created a 'Leviathan,' which wiped out large numbers of humans.
The rest of the world, though, helped to destroy the leviathan, and there was much rejoicing, as humanity was fighting back.
Some historians and scientists and the like discovered that the altar was actually a pad made by the aliens to create more leviathans, and then they discovered that they could create their own 'Titans' on the altars.
They created a Titan in Australia, then discovered more altars in Europe.