Carrying around microsd cards is great way to lose them/mix them up, but I see your point. Considering most phones allow up to 32 gb (where the Galaxy Nexus starts), it's not such a bad deal.
Most phones allow 32 GB, yes, but they also typically have at least 8 GB of on-board storage at this point, too. I've run into the situation several times where people have run out of space to store content on their phone, myself included. Allowing expandable memory solved my problem, and if I think I'll need it, I can pick up a Micro SD card with a variety of my movies and TV shows on it. For those other people who lack memory space, they have to go through all their things, manage what they want now, what they think they'll want later, and what they might not want, but might want to keep. Call me a pack-rat, but expandable memory is important to me when it comes to a major media device like phones have become. I don't want to rely on a cloud, or NetFlix, or an internet connection to access things, and any phone that doesn't allow expandable memory is a step back from what's on the market, iPhones and Galaxy Nexus included.