The value of a yakuhai pair: 1 han, easy to call or win on. Exception: If you aim for riichi and refuse to call, it may become dead. (But that is true of any pair.)
The value of a single yakuhai: One way to get a good shape (similar to a 2); no way to get a bad shape (dissimilar to a 2).
Nemata: "Usually, we think that 1 < single yakuhai < 2 < single renfonpai, but the influence of the rest of the hand is big." (Also: otakaze < 1)
Exception: 1 < otakaze < yakuhai
Redundant 1 or 2 due to very bad overlap.
Honitsu (i.e. honitsu chance, not necessarily confirmed)
Toitoi/chiitoi (if definitely chiitoi, then yakuhai < otakaze)
Big lead at or near all-last, save otakaze/yakuhai to help opponent cheap hands.
Saving safety tile in very bad haipai.
Already enough blocks?
Exception: otakaze < yakuhai < 1
Sticky iishanten, at or near.
Junchan
Sanshoku/ittsuu
Losing badly, need riichi, and not enough blocks.
Exception: Discard yakuhai first to deny pon. (Easier conditions for opponent renfonpai?)
Slow hand case: Losing badly, haipai needs tanyao and/or pinfu for mangan.
Fast hand case: Expensive hand losing substantial value if opened.
Pinfu looking for head