I know that the instrumental is used in structures of: 1) Means and manner; 2) Accompaniment; 3) Time.
However...
That's all well and good(ish), but Mitchell and Robinson (1986 ed., section 192) says Where there is no special instrumental form (and sometimes when there is), the dative serves. (my emphasis).
Does this simply mean "Even if there's a distinct instrumental form, you can get away with using the dative", or am I missing something subtle?