I think it's rather a nice thing to do - but I couldn't BF either of my DC and I would have been pretty gutted if someone sitting on the same bench as me breastfeeding her baby (when I was bottle feeding) had been handed a card and I hadn't, because I'd quite likely have felt that other people were watching me and thinking I wasn't doing a good enough job as a mother.
DS1 was born extremely prematurely. He was 14 weeks old before he could even try BFing, and every single time he tried he turned blue. But I expressed for 5 months, day and night, and he received what little BM I had in a bottle, mixed with formula.
DS2 could BF properly, but I didn't produce nearly enough milk to satisfy him. Don't ask me why, I went to our local BF Cafe, I asked for help from the nurses, everything. When he was 5 days old he was admitted to the children's ward with jaundice and severe weight loss, because I wasn't feeding him enough. So we switched to bottle-fed formula with all the drops of EBM that I could possibly produce added in.
So I tried. I tried so, so hard and I did my absolute best to give both of my children the best start they could possibly have. But to anyone walking past holding a "good for you for breastfeeding" card, I was just someone who couldn't be arsed and was therefore feeding formula from the bottle.