Answer
Recognizing birds as reptiles is useful since it provides an easy choice of outgroup for analyzing the birds phylogenetically and helps us to know ancestral character states. Recognizing them as a separate, equal group allows for a clearer analysis using the traits which are not shared with any living reptiles (such as feathers, flight).
Work Step by Step
First think about both groups (birds, reptiles). Features which are shared and can be studied in living examples could help if they are treated as one group, while dramatically different features (in living examples from these groups) are more useful for studying birds and reptiles as separate groups since the dramatically different features are hard to compare where one group is always null for those characters.
