Hmmm.. the only thing it really fixed was the licensing.  HBD was part of distributed in the 'toolkit' and as part of Svs V.  I think it was part of PWB 3.0, as we shipped it with RTU; but we have have gotten it via the toolkit license [I'm too lazy to look at Warren's System files to check].  

HDB was the huge fix of the original version that went 'wide' with V7 and BSD.   The primary changes were in house directories and the queues were handled, which had huge performance impact; particularly for large sites with an heavy UUCPnet load (a.k.a. the 'Usenet').   The other things that was in HDB was some collected alternate protocols besides Greg Chesson's original 'g' protocol; although those had all been distributed on the Usenet as net.noise, so all it really did is become a packaging thing.

There were a number of UUCP clones written in those days, and PC-uucp was popular for CP/M and later DOS systems that wanted to join the Usenet [Rick Adams, I think had a packaged version of some of them for his non-UNIX customers IIRC).    

As to why Dave Taylor decided to write another one in thoses, you'ld have to ask him; but in the end it replaced the V7 one in BSD.   But a lot of effort was made to make sure Taylor UUCP was more than a functional work-alike.   It had admin like HBD and use the same queues etc.     What I do not remember, Mary Ann might, is if Berkeley had been running HDB from the Toolkit on ucbvax internally, but could not have easily redistribute it ( seem to remember there were).   They funny part is that most Academics had a toolkit license by then becsuse they wanted HDB and ksh as a minimum, so most were running HDB; but the rules in the toolkit for academics were different than the original base license (I've forgotten them -- I had left academia so it did not effect me).   But it was around this time the Keith was start to try to remove code that  was knows to have AT&T copyright issues, when their was an alternative implementation that had BSD style licenses (which Taylor UUCP, as did Clark's troff as I recall).

BTW Grant, Linux picked up Taylor UUCP after BSD (it was not in the original distros and I suspect would have crashed the kernel in those days if the site was as all loaded).