That's potentially quite an expensive trade-off. You give up your ability to negotiate so that somebody else will collectively negotiate on your behalf.
That can work out well, at least for a while, but it's worth considering the longer-term consequences. E.g., in the longer term it can favour cheaper labour or cheaper competitors elsewhere. (Yes, you can argue that's the case even in the absence of unions, but there are stark examples of unionisation bringing about the premature demise or severe shrinkage of large industries: steel in the UK, the auto industry particularly in the UK (and also, to some extent, the US), coal in the UK, and the list goes on.)
For me outsourcing your negotiating power falls into a similar category to outsourcing your thinking: it simply doesn't feel like a good idea to me.