LAST MINUTE TICKETS! It does make sense to use miles when purchasing a last minute/short haul ticket. UA does not charge an extra fee for last minute award tickets. I say "short haul" because the miles you are losing by flying an award ticket are negligible.
Example:
A last minute SF0-BUR can cost $500 and would only earn you 1,000 status miles. Burn 25k miles and save the $500 for your next SFO-LHR (earn over 20k miles).