Update 2019-Jan-10
With v18, 3* 3.50 are now super easy to increase the special skill to 8/8 ( edit: see table in top post- Skill roll at Max tier, Max level ).
The only downside to using an off color trainer hero to level a 3* hero is the loss of 16.66% Hero XP, which is not inconsiderable when leveling 4* / 5* heroes, but, depending on your play style, may be better than buying hero roster slots.
Currently I am using 3* heroes to use up 1*/2* trainer heroes, unless I have a 4* hero that will be guaranteed 8/8 at 4* 3.60 or a 5* hero that will be guaranteed 8/8 at 5* 2.60 .