It would be imbalanced if assassination were more effective than other forms of PvP conflict.
A queen could die a couple of ways -- she might be murdered by a single stealth operative, or she might lead her troops in open warfare and fall on the field of battle.
In an RPG, there's no expectation that a single lost battle will bring a guild or monarchy to ruin. We would assume that losing a PvP fight is one chapter in an ongoing conflict that is really more about the glory of the fight itself than achieving regime change.
It's appropriate that successfully assassinating a queen would be about as effective as striking her down in battle, which is to say -- not all that effective.
Why bother, then? For the same reason that the two armies bother to ride into battle in an RPG: for glory, for bragging rights, for fun.
How cool would it be to slip back into your castle room, blood still fresh on your stiletto, and pen a cordial letter to your longtime foe: "Dear Queen Rowena, I was shocked and saddened this morning to learn of your recent demise. Who can that hooded miscreant in your chamber have been? And however did he manage to slip past your highly competent royal guard? Looking forward to continued warm relations between our great nations...."
But -- these deaths, however inconsequential to the game's balance of power, would have a real sting to them, because they'd cost real-world money. The next time Queen Rowena (now Rowena the Ninth) sees you, she's going to be thinking about that $20 she had to shell out for her premature Spark of Life renewal, and thinking of ways to make you pay the same.