The healing thing was definitely handing, especially when it came to Jessica and how she was capable of ripping someone limb to limb. She had seen the carnage left behind by Jessica and knew that death was instantaneous for some (especially those ripped right in half). If Peter had the ability to heal himself then it would be difficult for Jessica which gave him a great advantage over her. Though, because of the fact that Peter was telling all of this to Niki, he was also giving up this information to Jessica as well.
“You shouldn’t say anymore,” Niki added sternly, lifting her hand up as a means to silence the mimic. “Whatever else you can do is something you should keep to yourself. I don’t want her knowing everything about you. I don’t want Jessica to use it to her advantage. She’s difficult to handle and if she knows what you can do then she’ll expect it. If she…if she comes out I don’t want her to get the upper hand on you.”