Linksys appears to have done their homework on the 51AB's pricing. As I write this in early Nov 2002, the 51AB has a lowest web price of about $227, right under D-Link's DWL-6000AP pricing of $230. Given pricing of around $150 for an 11a-only AP (D-Link's DWL5000AP), plus the average going rate of $100 for an 11b AP, you save a little money by going with the dual-band box vs. buying two separate boxes. And that's really the question for the whole dual-band AP/router product category - is it better to buy combined or separate products?
Assuming, however, that you're in the one-box-dual-band camp, I'd say that the WAP51AB needs client monitoring capabilities and improved MAC-based association features to bring it up to where it needs to be.