It appears, though I haven't completed a very thorough survey of the languages yet, that the Slavic languages have done fairly well at preserving both the locative and the instrumental.
But... I still love Gaelic more than Lithuanian... one word: phonotactics!

(the broad-slender contrast is awesome and full of WIN

)
I will concede that Lithuanian has an impressive noun system. However: "...the loss of synthetic passive (which is hypothesized based on the more archaic though long-extinct Indo-European languages), synthetic perfect (formed via the means of reduplication) and aorist; forming subjunctive and imperative with the use of suffixes plus flexions as opposed to solely flections in , e. g., Ancient Greek; loss of the optative mood;..." ...clearly, Classical Greek has a more impressive verb system. (...though the use of infixes for the subjunctive is somewhat intriguing...)
(Posted on: May 03, 2010, 09:22:04 PM)
...also, also, as an aside, IIRC, "Let the locative live..." was more a response to the crazy (

no offense to any Finns: we're all crazy here

) Finnish practice of breaking the locative up into what seems like roughly a case per possible preposition.

Something the instrumental hasn't had to bear (it instead has a tendency to die and fuse with some other oblique case... still, so does the locative

).
((...and, yes, rest of the forum, we are total language nerds...))