MySQL: Add column to unique index or add new index?
For one particularly heavy select query, I need an index on
  (userid,name,valueclass).


This is a good reason to add an index. It would also be good to examine your query plan. BTW, underscores between words make variables more legible. 


  Are there any good reasons why I should NOT just add a column to the
  unique index I already have?


A unique index enforces uniqueness. If you have a need for the two column uniqueness then most likely adding a third column to that will compromise the integrity of your constraint. To assist your heavy select query the best option would be to add an index to (userid,name,valueclass) without the unique constraint.