Pluto,
Although I would not describe myself as a hobby astronomer, I think I can say that a good birding scope will open up possibilities for astronomical obsevations that no birding binocular can.
With standard eyepieces above about 25x you are going to be able to see Saturn's rings, cloud bands on Jupiter and its Galilean moons, impressive views of craters on the moon and, with an appropriate filter, sunspots.
I made up a filter using Baader Astro-Solar film for mv Swarovski ATM 65HD and unfortunately missed the Venus transit in 2012 due to bad weather (don't think I'm going to make it to the next one
) but did see the Mercury transit in 2016.
Many good birding scopes will cope with more magnification than with the standard eyepieces and I have used a 3,5 mm Televue Nagler with astro adapters on the Swaro (130x) and on my Kowa 883 (140x) to measure resolution. For observation though, it is important that the astro adapter attaches to the scope body and not the eyepiece, allowing a rapid change. I usually plop in a 28 mm RKE as a finder and then change to a shorter focal length for detailed observation. The Swaro has lots of in-focus but with some eyepieces I cannot achieve infinity focus on the Kowa.
Finally, the only possible disadvantage I see compared to similar sized astronomical refractors is some flare on bright objects. However, if you are looking for dual usage, the disadvantages of the latter for birding (weight, 90° reversed image viewing and lack of waterproofing) far outweigh this.
John