As far as I’m concerned the jury is still out here since this small study was terminated early, more patients in the montelukast group received magnesium and / or aminophylline, and it is unclear how the groups compared with regard to other other acute therapies such as beta-agonists and steroids.

BACKGROUND: Although leukotriene receptor antagonists have an established role in the management of patients with chronic asthma, their efficacy in an acute asthma exacerbation is not fully known.
METHODS: 87 adults with acute asthma requiring hospitalisation were randomly assigned to receive either montelukast 10 mg or placebo on admission and every evening thereafter for 4 weeks (when they were reviewed as outpatients). All patients were admitted under the care of a consultant chest physician and received full care for acute asthma according to the British Thoracic Society guidelines. The primary end point was the difference in peak expiratory flow (PEF) between active and placebo treatment the morning following admission.
RESULTS: Primary end point data were analysed for 73 patients. At study entry, patients who received montelukast (n=37) had a mean (±SD) PEF of 227.6 (±56.9) l/min (47.6% predicted) and those who received placebo (n=36) had a PEF of 240.3 (±99.8) l/min (49.6% predicted). The morning after admission, patients who received montelukast achieved a PEF of 389.6 (±109.7) l/min (81.4% predicted) compared with 332.3 (±124.9) l/min (69.8% predicted) for placebo (p=0.046). The mean difference between treatment groups was 57.4 l/min (95% CI of 1.15 to 113.6 l/min or 1.95-21.2% predicted).
CONCLUSION: In acute asthma exacerbations the additional administration of oral montelukast results in a significantly higher PEF the morning after admission than that achievable with current standard treatment.

Oral montelukast in acute asthma exacerbations: a randomised, double-blind, placebo-controlled trial
Thorax. 2011 Jan;66(1):7-11