Can you do the B-Spec Spa race without buying it? Either way, since there's a B-Spec event, I'm sure there will be A-Spec soon.
And no A-Spec doesn't make it useless. Arcade races are 100x better than A-Spec. The laps you want, the car you want, damage/grip/fuel settings, opponents that drive as properly as they're capable and don't park.
If only the difficulty changed the cars you race against instead of the idiocy level of the Artificial Idiots. That or just let the player select a field or class. In certain cars it's nearly(or just is) impossible to win outright on Pro against the superior cars you're pitted against. But put it on intermediate and you'll have a 10 second lead in less than 10 minutes of driving the same car on the same track against the same cars that were 5 seconds a lap faster than you last race. Then rubber banding kicks in.
Let us change our opponents, turn off rubberbanding, enable Weak slipstream(and perhaps penalties) and Arcade mode would be GT5 offline heaven.