There are two main reasons I bring my lunch to work:
1. It tastes better
2. It's cheaper
If I grabbed coffee and a breakfast sandwhich at TH's everymorning that would run me about $3/day. If I also got lunch each day it would average about $10/day. If there are twenty work days in a month = it will cost (at a minimum) $260/month to eat out.
My entire grocery budget is $200/month - so this doesn't work for me. That $200/month includes breakfast, lunch & dinner: Monday-Sunday. That is plenty enough money (for me) to eat in a month).
That said, I do budget $100/month for restaurants. If I choose to eat out a few times that fine, or if I want to go out for a fancy dinner, that's planned for too.
That's the point - it's planning. If you want to spend $260/month (or more) on eating your lunch out - do it. But plan for it! Know how much money you are spending and know that something else will likely suffer because of it.