Not really a trade-off. Players will find which piece of gear works best for each fight and use that. If the fight has too much damage going out that it can't be healed in that damage proc gear, you wouldn't wear that specific damage buffing gear.
And you keep mentioning "what if it doesn't proc". And? That fact that the proc exists places it above a piece of gear that doesn't have that proc, when used in the situation you are equipping it for.
In a comparison between two pieces of gear that are equal other than their procs, the procs on them being a % base is irrelevant when choosing between the 2.
