Linezolid is one of the last ditch antibiotics that can tackle resistant bacteria like MRSA. They usually don't prescribe it because of the high cost, which is artificially high to keep it from being over prescribed. Most of the time, either a standard broad spectrum antibiotic has failed or a culture and sensitivity test (takes about 3-4 days) has turned up a resistant bug.

While your doc might have acted with an overabundance of caution, foot wounds being notoriously hard to treat in diabetics and people with lousy circulation, the drug is usually prescribed only for resistant bacteria.

Just be glad you dodged a financial bomb.