heavies ability to tank is what matters the most for their role. In those regards , leanne is quite good , outperforming both bernard , and garret , and even marribel before her skill usage. Hell , maribel (which during her skill duration is comparable to a black unit) , will have 341 base defense X 2 = 682. Leanne , has 426 base armor , and assuming max skill level (you most likely won't have max skill level for a silver , but just for argument's sake , let's assume you do) will have extra 60%. That means 426 + 255,6 = 681,6. That is equal defense to marribel during her skill duration.
Ofc maribel gets also increased attack , and has naturally more hp , but in pure raw defense ability , leanne is right up there. Actually during her skill duration , she has more defense then even deine , but deine's is permanent (since she doesn't uses a skill) , and has monster like amounts of hp. The only heavies that can get more defense during their skill duration then leanne are gellius (which is the best heavy in the game for tanking purposes) and Bernice (which is the second tank in pure armor , during skill duration)
In conclusion leanne is a pretty good heavy , and even a maxed silver can be usefull , since it's cheaper and can be deployed faster then higher rarities. Also , since we're talking heavies , their role is to tank strong enemies , or purely be there for the block. Their dps is not important. This is the reason why garret is probably the worst heavy in the game