+ AoE magic attack to wipe out weak mobs so they don't tie up her blocking when something comes along you'd want her to block
+ AoE magic means she kills stuff faster than any heavy armor besides Deine, and maybe Garret (Maribel even during skill?)
- Less defense means she can't tank multiple heavy hitters compared to Deine, Gellius and skill-using Maribel

Since you said you only have silvers for heavy armors, the only time their defense would surpass Jessica's is during skill, while hers will be better (with more HP) the rest of the time, plus she won't take ages to kill stuff without ranged support like they would, so she's definitely worth it for you.