Rebalancing is pretty costly in my experience and also covers a different use-case than wanting to resize a channel. Resizing a channel allows me to maintain the balance I have with my channel partner while potentially adding new funds to send them. I could have a .5 btc channel and want to make 1 btc payments. Adding 1 btc to my channel is the only way to make that possible while still having capacity for regular payments as well. I personally think it's a huge improvement to what we had before