I think it is important. Less for relays and more for clients. Using gift wraps means that you're open to the possibility of getting DDOS'ed which messages that you have to decrypt to see if you're even interested in the content.

Having to send messages to many users (fanout) defeats one of the main reasons for using MLS in the first place. We want big groups to scale really well and be easy and fast for clients to update. In this way, they pull down only the events that they care about (for the groups that they're in) and only have to do a single decryption to get all the data they need to properly process the message content.