Singulair may help relieve some allergy symptoms (because inflammation is common in allergic reactions). I don't understand why your doctor would only prescribe Singulair for allergies - it can be safely taken with Claritin or Zyrtec and has a completely different mechanism of action than an antihistamine (which blocks the release of histamines that are a key part of the allergic reaction). Singulair takes a couple of weeks to reach its full effect in the body and must be taken every day to be effective.
There's no reason why you can't take Claritin or Zyrtec in addition to the Singulair (and in my opinion, if the over-the-counter Claritin and Zyrtec work for you, you probably don't need the more expensive prescription Singulair).