So I like the boeing jump bomber with its 9/14 a LOT. Its useful with 4 rockets as an 8/12 interceptor, and of course as a tactical bomber its great. Its just a useful, cheap, quick unit.
SI is a limiting factor on the slower fighters, as 50 tons mean unlike the brick-y assault aerospace fighters that get 10 SI on a 6/9 or 5/8 thrust, a 50 ton mechbuster can spend 5 thrust safely, thats it. So if you arnt going fast, but are heavy, it makes sense to be slow. Like, 3/5 at 50 tons as a cargo plane/cruise missile plane would be sweet, as you have standoff weapons... 5/8 speed on a 50 tonner is only useful in a 'redundant engine' kind of way for taking battle damage.
The other thing is if you play with the radar map, speed 10 is a critical break point. So, like the Light Strike Fighter hellraiser mentioned, a 10/15 conventional, is really important as part of an interceptor fleet to double move on the strategic radar map, or fly over the ground map each turn providing AA.