and I'd wager that Enterprise also had them for the glide tests. If memory serves, the ejection seats were removed after the 4th Shuttle flight because it really limited the crew size.

There was talk during the design phase about putting jet engines on the Shuttle for landing. However, someone accurately pointed out that X-15s flew 199 missions and every one ended in a dead-stick landing. So did the X-1s, X-2s, and other experimental aircraft like the lifting bodies. Some of those planes had a glide ratio that makes the Shuttle look like a sailplane. Knowing you only have one shot at landing tends to help the pilot concentrate.