Albuterol is used in acute exacerbations of COPD due to its characteristics as a beta 2 agonist, as you yourself stated COPD is most commonly diagnosed as emphysema and a chronic obstructive/ restrictive respiratory complication. In an acute exacerbation thereof the beta 2 effect is not directed at the emphysema as much as it is directed to asthma, bronchitis, or another obstructive/ restrictive disease, thus hopefully alleviating the resulting bronchconstriction and improving FEV1, peak flow, and VT. It should also be considered that diminished BS does not only mean emphysema as it is commonly seen in status asthmaticus where the bronchioles have constricted to a point where there is little to no airflow, in cases such as these bronchodilators are much more than just a placebo, they are immediately necessary.