How useful is an external flash?

I'm weighing whether to get an old Minolta flash for my Sony a580. However, I'm not quite sure how useful an external flash will be.

My intended uses:

• Playing with high speed photography (flash on lowest power for short flash duration, and firing the flash with the shutter open in a dark room to freeze something)

• Handling movement in very low light situations, where even ISO 12800 and f/1.7 fails to give an adequate shutter speed.

• Miscellaneous studio style photography in a school photo club (not quite sure what that will involve, or even if a flash will be useful there)

Possible uses:

• An oversized AF assist lamp

• Lighting backlit subjects so I don't have to use fill light or tone curve adjustments (if HSS works correctly)

Here are my main concerns about an external flash:

• Firing a flash attracts plenty of unwanted attention, and flash is prohibited in many situations.

• The flash's light color often differs from the color of ambient light, making white balance adjustments a nightmare. Fumbling around with a batch of color filters (and remembering to change that every time the lighting changes) seems like a clumsy and error prone solution.

• Adding light to a scene with a flash tends to change the 'mood' of the shot.

• The effective range of the flash is limited

• If an external flash proves cumbersome to carry, there's a good chance that it won't be with me when I need it

Based on this (and your experience), would spending $40-80 on an external flash be a wise decision? Thanks!