What a great question, R.! As a mother of an only and a grandmother of an only, here are a few of the fabulous bennies of being a mother (or granny) to one:
1. Even relatively limited financial, personal, and energy resources can work, and work well. I don't have to spread myself, my budget, or my time too thin in an attempt to meet all my children's needs. Inevitable emergencies will have less impact, because fewer family members will be affected.
2. Life at home is surely calmer with one than with multiples. No sibling rivalry issues, no fighting, less noise, less mess. More adult attention when needed. And I have extra time to really research optimal child-rearing. I'm always interested in polishing my chops.
3. I can really focus on the unique and special qualities of one child, and come up with the best possible educational, cultural and social opportunities. You should see the fabulous projects and games my grandboy and I do together!
4. I have extra time and energy to share with other children, many of whom desperately need caring and available adults in their lives. This is important to keep in mind because those children, along with whatever deficits they may carry into their adult years, will be part of the world MY child and MY grandson have to share.
Over the years this has included volunteering at an elementary school, teaching kids' classes at my church, and offering free babysitting to stressed moms.
5. I don't want my personal desires to cloud the futures of children living today. More people = more impact on the natural world, upon which all life, not just human, ultimately depends. We already have a human population that's crowding out other species, polluting, and over-consuming natural resources, and those problems have become ever more evident since I had my daughter in 1971. So for me, limiting family size is one of the greatest gifts we can give to the future.
Those are just the top 5. I'll bet I can think of at least a dozen more good reasons without digging too deeply.