They don't use Cell because they know he would be broken AF. Seriously, back in the Android (can we please start saying Cyborg instead which is more appropriate?) saga he was what would be to a normal Saiyan SSJ1 and still beat their asses like they were nothing even before reaching that level.
Now just imagine him reaching SSJ Blue level... He would break the curve all over again. At that point they would need Gohan to surpass Mystic (which already can keep up extremely well with SSJ Blue)... You know what, please do being him back just for Gohan to get a new form.
