Let's be honest here, Astrid is a bow-locked level 1 unit coming in chapter 13. Sure, she has Paragon, which means that level deficit won't be around for long, but she's still bow-locked until promotion. Having a horse means that she can hit and run with no real risk, but so can Oscar, Titania, Kieran, Marcia and Jill and, to a lesser extent, Makalov, who joins the next chapter (well, not with no risk, but you know what I mean). Not to mention that Marcia and Jill can, y'know, FLY. What she brings to the table can be done better by any of the five units I just mentioned, though having another horsie around never hurts (apart from the obligatory desert chapter).
6.75/10, TL;DR, she has potential, but not much more than the other mounts.