i am not a mesa expert, but if my understanding about Gallium is correct
Gallium backend is shared between different drivers.
if intel use Gallium, they will need to tune the Gallium backend performance and implement various state tracker (e.g a full functional vaapi/vdpau state tracker).
But this means that AMD driver will be benefited from these features, too (automatically)
AMD developers will have more time to tune their driver and it will mature much faster.
Currently, intel is no match with AMD for display hardware. If their driver are at the same level, AMD kicks intel's ass.