Smart phones and tablets shouldn't need transformers. That would be unnecessary weight to cart around. That said, the devices that allow you to plug in four USB charging cords can be quite heavy themselves. I'd recommend comparing weights in online listings as well as reading a bunch of reviews. There are some poorly-made electronic devices out there.
I suppose the safest thing to do is buy a simple adapter plug (probably $1 to $3) and use it with the charging cord/plug you received with your electronic device. However, between the two of you, you probably have multiple devices that need to be charged every day. Many European hotel rooms have few available outlets, so having a single device to which you can connect four USB charging cords is very handy.