To mow or not to mow?

I've been letting my grass grow unmowed for the last three weeks and I'm actually enjoying the thick, green, untamed, healthy look. I haven't watered outside of the generous rain that's been falling (twice the normal annual average), and the lawn is largely more shaded than sunlit, though every part gets at least a little sun in the day. The grass is a mix of Scotts tall, fine and red fescue, bluegrass and ryegrass. I fertilized once in the early spring along with the seasonal overseeding and sprayed one application of Ortho Weed-B-Gon three weeks ago to fight the weeds leaking in from my neighbors' unkempt, patchy low-mowed lawns on either side. I generally mow with a manual Craftsman push mower at about 2.5 to 3 inches height, except for the last three weeks. The grass stalks are starting to grow seeds at the top now.

Is there a reason, other than cosmetic, for me to go back to mowing the lawn?

I've decided to do the same thing, just let nature take it's course, and let the grass grow freely.

At some point, I'll cut it, but the purpose is not for aesthetics, but for creating mulch.