Well, your ESXi host will have some overhead, as well as each VM. You can over-allocate though.
As for advantage/disadvantage to allocating it all, the advantage is obviously that your VM can utilize more RAM. The disadvantage is that if you end up over-allocated and your VMs use their full capacity simultaneously, anything above the overhead+utilized RAM would be swapped to disk which is a huge performance hit.
However, through Transparent Page Sharing, ballooning, etc. you would likely be able to allocate 12GB to each VM and never run into this issue. The safe thing to do however is reserve ~2GB for the host and VM overhead (you can tweak this to match your environment's actual overhead at any time). This might be more than needed in some situations, but it should keep you from ever over-allocating.
TL;DR - Allocate up to 46GB, since it seems likely your VMs won't even use the amount you're giving them, and modify VMs if you notice a trend of too much/too little RAM allocated to them.
~Brandit