Rommath is only a "who" to people who are unfamiliar with Sin'dorei lore; he hasn't gotten a great deal of attention in the game, but he's actually a hugely important figure to the Blood Elves. As Grand Magister of Quel'thalas, Rommath is the leader of all Blood Elven magi, and was part of the company of Blood Elves who went to Outland with Kael. He was extremely loyal to Kael, actually; he was sent back to Silvermoon as Kael's herald, telling everyone about Kael's plans to build a paradise, etc (obviously, he wasn't in on the crazy). Not only that, but basically everything you see in Silvermoon in terms of reconstruction you can thank Rommath for. He was the one that rallied the mages to help rebuild the city, to beat back the Scourge, etc. Honestly, he's probably more of a leader than Lor'themar's been, it just hasn't been broadcast that much.

So him turning traitor is every bit as demoralizing as Benedictus. Maybe not to the entirety of the Horde, but certainly to the Sin'dorei as a people, and just a few years after the Sun King's betrayal. Though, I will point out that the Blood Elves are arguably the most magically adept race around, and losing the guy who was essentially in charge of that magic is bound to hurt the Horde.