Odds and ends.
These don't particularly belong anywhere and so here is the post for them :)
As a Myrmidon mechanic, he adds some useful buffs to a RoS player. Firstly, our Casters are not renowned for their harmony with the Myrmidons, especially when compared to some Factions out there. The Arcanist can ease some of the focus strain from our Casters, allowing them to perform the role they are designed for, and so giving the most benefit to the army. Secondly, our Myrmidons are notoriously underdone in mêlée, perhaps as you'd expect for a Generalist 'jack, and so the Arcanist's damage increase spell can be handy. A well thought of Solo.
This is a unit attachment, and gives some anti-magic protection to what it is joined to. When combined with flank: unit the Escort becomes a solid choice, if not a must have. This choice seems more dependent on your local gaming situation and what your mates are running than anything else, and so is not universally as lauded as the Arcanist. If you have a big unit to attach the Escort to, the value from its protection increases both in terms of number of friendly models protected, but also the priority the opponent will allocate against taking out said unit in the first place.
Both of these odds and ends choices are quite good, and help augment most RoS armies.