I'm going to keep this short and simple.
If you function as a mom, you deserve recognition as a mom. If the mom who gave birth and the stepmom both care for the child, they both deserve some form of recognition.
The mom will always have the all powerful bond and genetics and history and title and...and...and...more. If the stepmom functions as a mom in some way, giving of her life to raise the children as well, she can still deserve a call with a "Happy Mother's Day!", a card, a gift, a dinner. She will always and forever lack all the rest, from birth on, but she does deserve recognition for the work, effort, presence, guidance, and crap she puts up with from pure dedication. She may not seem perfect, act like "mommy", or be the Sound of Music (wannabe) nun, but she is there in their lives, serving as a mom in some capacity. And such, given the role she takes on, deserves some recognition on Mother's Day.
We honor our grandmas, friends who are mothers, and our friend's moms on mother's day. It's really not that hard to accept that the woman who also cooks, cleans, and cares for the kids gets some form of kudos on Mother's Day, as well. She fits in the category, by definition past birth, and anyone who says otherwise needs to examine where their judgement is truly coming from.
Oh, and cut that crap out, jealous moms. We'll always get the lesser gift, the forced "happy mother's day" wishes, and the afterthought. You won at the pregnancy and we don't know why you don't get that still. Be proud to be their real mom, and we wish you would be by showing your kids how self-assured and truly loving and accepting you are- by accepting us.