Of course, there are occasions when toddlers are not welcome- dental appointments, massages, etc. And for these times I get childcare. But, when possible, I choose to bring my son with me because it is typically more enjoyable for me and I think he actually gets something out of the experience.
During these outings he is learning how to be a part of a community - a community not focused on him (unlike a playgroup or a music class). He's learning that it is fun to say "hi" and "bye-bye" to the nurses, cashiers, waiters, and doulas that we interact with. That he must wait while mommy fills out paperwork, talks with her doctor, stands in line or gets her blood pressure taken. And that there are times when he must find his own way to entertain himself (a skill that will be important when baby #2 arrives).
Personally, I choose to take my son on these outings because it's fun to see the world through a toddler's eyes. He says "hi" to a biker waiting at a stop sign and eagerly waits for a reply. Or stares awestruck at a construction site across the street. He also points out things I wouldn't have taken notice of - the firetruck in the parking lot, a dog and its owner playing frisbee or the stadium on the cover of a magazine.
Do you bring your kids with you when running errands? If so, do you think they benefit from the experience?