First of all, I don't think relying on the Arcane Art ability is a BAD thing. It's powerful, versatile, and the Bard's most important and useful ability in combat. Still, I understand that sometimes it's TOO powerful an option, and in those situations there are a couple of excellent ways to deal with it.

First, as Munin and noclue noted it's absolutely okay for the GM to declare that the Bard has drawn a lot of enemy 'threat' from their casting (should they roll a 7-9). Draw unwanted attention tends to be a go-to choice for some players, after all they're already fighting so what's the harm in drawing a little attention? The solution is reinforcements and/or traps being prepared.

Second, another possible consequence of Arcane Art is having the magic affect unwanted targets. Perhaps that healing spell doesn't just affect the Paladin, but also the three orcs he's fighting. Maybe it breaks the sleep spell the Wizard cast on the ogres outside the cavern, and now they're hankering for some revenge. There's a decent amount of leeway with the ability, enough for a canny GM to exploit the roll.

Regardless, even for a Charisma 18 Bard, getting a 10+ on an Arcane Art roll is still only about 40%. While the ability CAN be incredibly useful, especially when healing characters during 'down-time', it's not consistent enough to be 'spammed' without long-term consequences.