In my understand spirit index was intended to keep mages from assembling a spirit strike team. The wording is vague enough to let GMs be more aggressive with it, if they want too. The Edge vs summoning being the sort of classic hinge example. So long as a caster does the expect run thing there shouldn't really be a problem, and should one develop the wording is also such that getting out from isn't actually hard, it just takes time and a some minimal effort.
On a side note I'm very surprised we have never seen a piece of shadowrun fiction that had single mage be a whole team via the use of spirits.
But that's not really important, spirits can be strong no question, and a smart player can get a lot out of one. However I still wouldn't call it game breaking issue. Really from GM perspective spirits are lot better for the NPC side then the player Side. Players who push to far, spirits are the perfect tool for checking that sort of issue. Their powers scales very easily and predictably, some of them really subtle and some are really not.
Just in closing to the OP, this really should be obvious, but clearly you missed that day in school, never suggest a magic or any other a rules change with a justification that includes "Fuck Subject of the Rules Change". No one will ever take you seriously, it utterly destroy any credibility your argument may have had.