What do you use a med/lt bait caster with 10lb line on a 5'4" rod for? It's a light & medium but more on the flexible side.

That sounds like too heavy of a line for the rod. I would use 4-6 lb line and target perch or small bass.

I purchased a zebco 5'4" AVSv1.0 ZAVSC542ML 6-12. Line 1/8-1/4oz lure. My reel is the Chinese bait caster DM-120RA from the auction site. It feels like a well balanced setup. I changed the line to Cajun 8lb test. The rods look awesome. The reel feels like a more expensive reel with 10 ball bearings. Time will tell when it has some use on it. But right now I'm sure the $34 price tag will go up in the future.

I have a Phenix recon PHX-C682L, it's 6'8" light paired with a Shimano curado 50e with 8lb fluorocarbon. I mostly just use it for small weightless plastics and sometimes dropshot or shakyhead. 

 Dobyns Extreme Cast Rod 7' Light 701 paired with an Abu Garcia LTX and a St Croix Legend Xtreme  6'8 Medium XF paired with an Abu Garcia LTZ, both are extremely fun to use in the summer for finicky biters. 

Switch to 20 pound braid and cast small crankbaits with it.