If someone plans to set up his/her own shop some day definitely, then is does it make sense for him/her to attend graduate school just so that he/she may have an awesome social network? Considering the hefty student loans that need to be repayed asap that accompany graduation....
Yes, it's foolish. People can disagree about how much value there is in a MBA, but that's not the point. You've identified two targets - being an entrepreneur and building out your network. Go straight for the jugular, not some round about way through school.
Want to build your network? Take half the money you would've spent on grad school and support yourself while you explore early business opportunities and network like mad.
It's quite simple. Want to build your network? Spend your time, money and energy networking in the right places. Going to school for networking only is like going grocery shopping every day hoping you'll meet your future spouse in the aisles.
Taking it to the extreme... take that money and throw massive parties, sponsor relevant meetups, make a $5k investment in a startup, etc.
[Source: I dropped out of school with only a few courses left to found my startup that became a VC backed Silicon Valley company.]