Thinking back, it isn't even money. My lawnmower and string trimmer were a housewarming present from my FIL when we moved into our first house (total cost maybe $500). I added a blower attachment and edger attachment for the weedwacker, each ~$50-75. And I spend about $20/yr on gas to run it all. So what's the big deal?? There is no money savings, figure I take care of the yard 1/2 the year (not in winter) I'm at about $1/wk to buy 1-2 hours of weekend back. What kind of fool doesn't take that payback?!?
I'd take it, and most others probably would, too..."Mustachian" or not.
I think there are some good counter-arguments from MMM and others, but only you can tell if they really apply to you:
1) It's exercise...people pay upwards of $50 a month for gym membership to get exercise, so why not get it in the form of yard work (the nature of yard work would force you to exercise regularly as well as to get outdoors)? May or may not apply to any given individual depending on how they spend the rest of their time, their physical condition, etc.
2) If you don't want to mow a lawn, then don't have one. I am assuming you have a good reason for having a lawn. As it turns out, we didn't, so we got rid of it. I didn't particularly care for it, it attracted mosquitoes en masse, it was in the front yard where we don't go for recreation, and the effect on my water bill was several hundred dollars per year. Some people actually utilize their lawns, so this decision would be more difficult...for us it was easy.
3) Are you really doing something more valuable with the time saved? For you, it sounds like you enjoy family time on the weekends when you aren't cutting grass by hand...good for you, and an easy decision I think. If the alternative was sitting in front of the idiot box watching reruns, I would advise you to cut the grass by hand.
Even MMM uses a range of pneumatic and cordless (woah, talk about luxury!) tools, 2 cars, electric bikes, a clothes washer, a gas oven (threw these last two in to ward off any "yeah, but those are just for work/blog purposes" comments), and a host of other things that cost money but save time and effort. I think the message is not following this to the extreme, and to avoid cutting out all discomfort from your life. Mowing the lawn with powered tools is not necessarily harmful in isolation, but if you extend this logic to every other decision and end up on your ass all day moving from chair to chair and enjoying how 'convenient' your toxic life has become, you've got a problem. Only you can judge where you fall on that continuum, and in the meantime I'm just happy to have fellow forum-goers who will challenge each other on this.