Here's how I see the whole "ruining a player by bringing him up to early" argument:
a) If a player has NHL level defence, but weak offence, and you think he can develop better offence given time in the minors, then send him down. In the NHL he'll probably fall back on keeping it simple offensively, while playing good d, and will thus develop into a role player, not a star.
b) If a player has NHL level defence, and weak offence, BUT you're projecting him as just a defensive player anyways, then keep him up as long as his NHL impact is positive.
c) If a player has NHL level offence, but weak defence, he won't be hurt by playing in the NHL, but he might hurt your team with his weak defence. As long as he's helping your team, then keep him up.
And then a final note:
b) If you think a player is going to get physically ruined in the NHL, and quickly injured, leave him in the minors until he bulks up and/or learns to avoid big hits, regardless of his offensive or defensive ability.
I think Rielly falls into category "c)", so I'd keep him up as long as he has a positive impact, and as long as you're not worried about "d)" (high injury risk). With that being said, I'm not sure that he's currently good enough to have a positive NHL impact. So I voted "Play him for 5 games - then re-evaluate at that point."