[Wahoo73]

Interestingly enough, a very good poker pal of mine and I were discussing this very subject last night, inasmuch as we both play a lot of stud at levels from 1/2 to 5/10.

Here is what I have concluded about these (insert your favorite poker player derogative here):

Their MINIMUM starting hand requirement is any pair (regardless of kicker and regardless of what other cards are on the board), or any two suited cards (regardless of what other cards are on the board), or any three cards that are connectable (regardless of whether they are one-gapped or two-gapped and regardless of what other cards are on the board). I estimate that at least 1/2 - 2/3 of all the possible starting hands in stud fit these MINIMUM requirements.

Once they have decided to play a hand, they then will play almost every hand to 5th street. If they pay any attention to what appears in other players' hands it's only to see if they show open pairs. Then, and only then, will they possibly fold on 4th street. Otherwise they play till 5th street and then reevaluate their own hand (and only their own hand) to see if they have any pairs or any draws. If they do, they continue, inevitably to the river.

Your posted hand is a prime example of this. Villain has two diamonds in the hole to start so he plays it, regardless of the action. He then catches another diamond on 4th that also gives him a 3-straight, so he continues, regardless of the action (and after all, there are no pairs showing). On 5th street, he picks up a gutshot straight draw (the favorite hand of these types of players and there are still no pairs visible), plus he still has his 3-flush, so he plays on. 6th street is nirvana for him, as he picks up that 4th diamond and calling is a no-brainer at this point. The river answers his prayers. And so it goes, thus validating his inferior starting hand requirements and his strategy to play any pot he enters to at least 5th street.