It is necessary even for distance runners to work on speed.  I feel it gets your body back into producing fast acting energy.  It is not my favorite workout!  We need to fine tune the short and long supply of energy stores.  Speed is really not a goal in Ultra running, it is finishing.  You are really not in competition with anyone but yourself.  If we could remember this on everyday living, life really would not be that demanding.

Keep thy heart with all diligence; for out of it are the passions of life.

~ Proverbs 4:30