That depends on whether or not you consider short rests for healing important things where that time limitation makes an important difference or not. In some campaigns that wont be. in others it will be. But by this logic of "so what" would you be fine if it just said "everybody goes to full" and skipped the dice by dice effect? They both would just "gives you a short rest worth of healing at the cost of a spell slot rather than time and HD. Cool. So what?" Removing "short rest" is the same regardless, right, no sense to need it to be scaled upward, right?
A wild leap, but I’ll humor you this time.
So, that isn’t as easy to parse as you might think, so here’s what I think you’re saying.
*If a 2nd level spell that
can bring everyone up to full* is no big deal, then making the spell simply heal everyone to full would also be fine. Since that obviously isn’t fine, the spell is a big deal.*
That doesn’t follow, at all. An equivalent argument would be, “because Expertise can sometimes make it so that only a roll of 1 can fail on some checks, it might as well just say “you automatically succeed on all checks with this skill. Clearly, therefor, expertise is broken.”
* (note that I said “fewer/no”, recognizing that the dice make it uncertain how much healing the party will get, and the vagaries of combat make it uncertain if that amount of healing will be enough)
Because there is a significant difference between “has a chance to” and “does, automatically and without chance for failure to do so”.
However, a spell that did guarantee full health, with a casting time that precluded its use in combat, wouldn’t be a huge deal, as long as it’s at least 2nd level. If it has a combat useful function, it would depend on that to determine what level it should be.
Healing Spirit, at worst, could be said to be too powerful for 2nd level. I’d consider moving it to 3rd level, at the highest. It saves the group some HD. That isn’t a big deal.
edit: also, as [MENTION=6795602]FrogReaver[/MENTION] points out, it would have to scale with up-casting, in order to be even kind of vaguely analogous.