Another problem is that phonetic transcription works on the assumption that the speech signal can be divided into single, clearly demarcated "sounds", while in fact, these "sounds" do not have such clear boundaries, therefore what phonetic transcription takes to be the same sound, might be different according to context.
Nevertheless, phonetically transcribed corpora is extremely useful to the linguist who lacks the technological tools and expertise for the laboratory analysis of recorded speech. One such example is the MARSEC corpus (which is derived from the Lancaster/IBM Spoken English Corpus) and has been manipulated by the Universities of Lancaster and Leeds. The MARSEC corpus will include a phonetic transcription.