As jakewash implies using a phantom center is not ideal. I used a phantom center with my M80s for quite some time and found it worked very well. Beside the drawbacks mentioned another is that you can’t separately control the center channel’s volume without a dedicated center speaker. This is a pain in some movies where the L/R channels overpower the dialogue.
Advantages I found using a phantom center are that it simulates having three identical speakers up front so the whole front soundstage has exactly the same sonic signature. Another is that forgoing a center channel for a little while lets you put that money toward getting better L/R mains and adding the center speaker later when funds are available. However, I wouldn’t generally recommend not ever getting a center speaker it’s just to integral to HT and multi-channel music.
