If that's true, then gems like that are more suited for characters like Minoriko, Cirno, and Chen, all of whom has low stats and a high leveling rate(bonus points for Chen having the fastest leveling rate in the game, so it would work rather well on her).
Kinda... Minoriko's magic is high enough as is to heal most characters to full, cirno is more for slow and par than damage (or did that change? I forget). Chen is absolutely a great candidate though. Mind you her def/mnd is so low that even with stones she'll probably be 1shot by many things.
Stats need not be bad to take good advantage though. Nitori for example is a character with great power, but relatively low stats (before shooting sky high due to maintnence and good equipment). This is because her formulas are her strong point in her attack rather than her stats, soo.. Yeah. That and the value of gems kinda double after maintnence since gear multiplies your stats, which have now been affected by the gems.
That all said, gems are nice, and "better" in those cases, but they really dont make or break characters, because even with max use, the base stats on a character, even if low, are going to outperform the gems, unless we're talking about yuugi's mnd or something.
Defensive gems work the sme way but due to the fact that reducing damage by 1000 when you only took 1100 is a much bigger deal than reducing hits by 1000 when you take 10000, having them on someone with good defenses is probably not a less beneficial choice at all.
Speed gems on people like patchy are no brainers imo though. And yeah, atk gem and chen is excellent as well since shes another case of unspectacular stats with great formulas.