OK ... I'll give it a shot. Short answer: Yes, a finishing sander is good to have since it's (primarily forward/backward) motion makes it easier to sand one side of an inside-corner without gouging into the adjacent side (common with a round random-orbital).
For vibratory sanders, there are 3 types that I know of:
1. Random-orbital. The sander moves the sanding head in an elliptical/orbital motion AND spins the head at the same time ... fairly random, good for avoiding cross-grain scratches on wood (not an issue for sanding epoxy)
2. Orbital (like the Makita you've referenced). The sander moves the pad in an ellipotical/orbital motion, but no spin. Since it's not directly forward/backward motion, it is less likely to leave cross-grain scratches on wood. Again, not an issue for epoxy.
3. Forward/backward pad motion. Motion is not elliptical or orbital. Requires that you keep it moving to blend areas as you sand.
Except for very fine sanding, I would avoid sanders that only spin or use driven belts of sandpaper since (unless you need aggressive sanding/shaping). I like random-orbital the best and use it for almost everything. As mentioned above, the non-random orbitals (most finishing sanders) are good for along an inside corner and help prevent sanding damage to the adjacent side. All sanders need a degree of skill, but orbital or random-orbital are easiest to use.
Brian
