As of right now, there's no incentive to ever reroll with legendary/ultimate shards because they don't net you anything better than any other skin shard. I was thinking of a couple possibilities to make use of them. Implementing one or both would be a neat way to make rerolling your high-value shards worth it.
1. Increased chance of getting a legendary/ultimate skin, up to 100% if three are used.
2. A healthy dose of bonus cosmetic essence on top of the normal chance to get 100.