Problem w/ excess mucus; slight wheezing if exercising hard. Doc says it's likely allergies. Why prescribe potentially dangerous drug like Albuterol?

Asthma. Allergies can trigger asthma. Antihistamines will not help asthma.. Albuterol is a first line medication to relieve the spasms of airway muscle that occurs with asthma. It is an excellent medication for occasional use. If albuterol helps but you need it more than twice a week, a preventative medication may be needed. If albuterol does not help, then other causes of symptoms need to be considered.
See an allergist. Albuterol, although safe, is not necessarily the best answer. Your symptoms sound like asthma which is triggered by allergies 80% of the time. See a board-certifeid allergist to learn what may be the best approach. Albuterol is a reliever short-acting bronchodilator that is safe when used prior to exercise and only if needed. You may benefit from being on a regular controller asthma medication .