Doing open world content will provide minimal help to tanking and healing, because all open world leveling content are DPS orientated. The only difference between new jobs at 15 and at 30 is where you encounter newbie tanks and healers, not whether or not you encounter them. Remember all those newbie Gladiators and Marauders in Sastasha? Grinding level 1-15 didn't seem to help them at all.
You do have a point when it comes to absolute newbies to the MMORPG genre. Introducing 1 spell/skill at a time is valuable for people who are so new that things like forming a party, things like upgrading gear, etc., all starts to overwhelm them.
Besides MMORPG newbies, forcing the grind only has the effect that people who are serious players will have time to think about each skill; but people who won't think about their skills still won't even if you force them to grind an old job to 50. Look at all those level 50s who have extremely wrong ideas about their skills. That 50 level grind where they received skills one by one didn't help one bit, did it?
So while leveling will help some, it won't completely stop those kind of players from entering the queue. Forcing the grind from 1 doesn't feel so effective if your main reason is to make people learn their jobs---as is already proven by the DF right this moment.