#3. Finn Balor shouldn't be added: It could affect the marketability of the SummerSlam main event
As great as Triple Threat matches in WWE are, Finn Balor being added to the SummerSlam main event might affect its marketability. Roman Reigns vs. John Cena is the perfect match to market for the main event of a pay-per-view like SummerSlam, or even WrestleMania for that matter.
That's why it's a bit surprising that the company had a Reigns vs. Cena match at No Mercy 2017, not even putting it as the main event. Instead, they put the Universal title bout between Brock Lesnar and Braun Strowman as the top match for that pay-per-view.
Perhaps WWE wants fans to forget about it, which is why they've obviously avoided any mention of it in the build up to The Biggest Party of the Summer.
Finn Balor might deserve to headline SummerSlam this year, but it doesn't mean he should. From a casual fan viewpoint, a singles match between Reigns and Cena is far more enticing than a Triple Threat involving Balor.
A Triple Threat with Finn Balor would most likely lead to a better match in terms of quality, but it's only a smaller, core fanbase that truly cares about this. In reality, a larger chunk of fans would prefer the main event of SummerSlam without The Prince.