Is it okay to protect NM Cable in an unfinished basement with ENT?
You'll have to check with the Authority Having Jurisdiction, but I'd say "smurf tube" on a wall in a shop is a no no.

National Electrical code allows ENT for exposed work, but not where subject to physical damage.  Which means if you're trying to protect your cable from physical damage, you can't use ENT (unless you protect the ENT from physical damage as well).


  National Electrical Code 2014
  
  Chapter 3 Wiring Methods and Materials
  
  Article 362 Electrical Nonmetallic Tubing: Type ENT
  
  II. Installation
  
  362.10 Uses Permitted.
  
  1) In any building not exceeding three floors above grade as follows:
  
  a. For exposed work, where not prohibited by 362.12
  b. Concealed within walls, floors, and ceilings
  
  362.12 Uses Not Permitted. ENT shall not be used in the following:
  
  (9) Where subject to physical damage.


If you don't want to use metal conduit, schedule 80 PVC conduit is identified for areas of physical damage.


  Article 352 Rigid Polyvinyl Chloride Conduit: Type PVC
  
  II. Installation
  
  352.10 Uses Permitted.
  
  (F) Exposed. PVC conduit shall be permitted for exposed work. PVC conduit used exposed in areas of physical damage shall be identified for the use.
  
  Informational Note: PVC Conduit, Type Schedule 80, is identified for areas of physical damage.