Morris Halle (1959) The Sound Pattern of Russian pp 20-24 (i.e. section 1.3)

Segments and boundaries are theoretical constructs and must, therefore, be appropriately related to the observable data. ... it must be possible to read phonological representations regardless of whether or not their meaning, grammatical structure etc is known to the reader.

Condition (3): A phonological description must provide a method for inferring (deriving) from every phonological representation the utterance symbolized, without recourse to information not contained in the phonological representation. [Synthesis requirement]

Condition (3a): A phonological description must include instructions for inferring (deriving) the proper phonological representation of any speech event, without recourse to information not contained in the physical signal. [Analysis requirement]

1.31. Condition (3a) can be met most simply by ... one sound per symbol and one symbol per sound ... As is well known, all attempts to implement this slogan failed because they invariably resulted in an apparently limitless proliferation of symbols

some sort of limitation on the number of symbols

1.32. Condition (3a-1): Only utterances which are different are to be represented by different sequences of symbols.

it has been proposed to require that positional variants of a particular phoneme be "phonetically similar". Unfortunately this only serves to push the difficult one step further, viz., to the question of what is meant by "phonetically similar"

1.33. In Russian, voicing is distinctive for all obstruents except /c/ (i.e. /ts/), /č/ (i.e. /t/) and /x/... These three obstruents are voiceless unless followed by a voiced obstruent, in which case they are voiced. At the end of the word, however, this is true of all Russian obstruents.

E.g. [m'ok l,i] "was (he) getting wet?", but [m'og bi] "were (he) getting wet"
[ž'eč l,i] "should one burn?", but [ž'edž bi] "were one to burn"

In satisfying both Condition (3) and (3a): /m'ok l,i/, /m'og bi/, /ž'eč l,i/, /ž'eč bi/ (phonemic representations) + a rule stating that /c/, /č/ and /x/ are voiced before voiced obstruents.

If Condition (3a) is dropped: {m'ok l,i}, {m'ok bi}, {ž'eč l,i}, {ž'eč bi} (morphophonemic representations), and the rule would cover all obstruents, not just /c/, /č/ and /x/. Condition (3a) involves a significant increase in the complexity of the representation.

the morphophonemic representation and the rule suffice to account for all observed facts. Phonemic representations, therefore, constitute an additional level of representation made necessary only by the attempt to satisfy Condition (3a) . If Condition (3a) can be dispensed with, then there is also no need for the 'phonemic' representation.