Heh, I pointed it out to that guy, personally, that multiple individuals' scrutiny had effectively overcome the answer of, 'summoners can summon because.' I'm not against people pointing out RAW rulings to those who had misread, but the army of interpreters has been frustrating that thread.
I think summoning can have a happy marriage in the end:
1) Summoners can summon whatever they please if they have the keywords necessary, regardless of whether the unit is on the table.
2) Summon spell titles earn a colon after the word, 'Summon.' For example, 'Summon: Saurus Knights.' This designates 'Summon' itself as a spell, meaning that it may only be cast once per turn (unless otherwise stated).
3) Debatable: armies/models that 'need' summoning earn individualized exceptions based on the models being summoned.
4) Ridiculously powerful summoners (like Nagash) earn a keyword, 'Legendary,' which - due to the scale of their power - restricts that player from accessing the Sudden Death table.
I do hope it gets reined in somehow.