The major trouble with having same-identity alts elsewhere is keeping them in sync. You have to pass essential parts of your inventory back and forth, or if something is no-copy or even no-transfer, you have to get it once per avatar. If you aren't one of those guys who always wear the same, you have to create your outfits several times over. And you have separate friends lists, so all your avatars have to befriend the same people.
It's also kind of ironic that grid outages inspire people to create alts on different grids and at the same time keep them from actually doing it. They'd have to create their alt from scratch, for as long as their home grid is down, they can't access their main avatars and give their alts things from their inventory such as the shape.
Might be the reason why many people have alts that look vastly different from one another while having the same name.