When you want to practice a language.
If you want to challenge your language skills with complete immersion, hiring a guide who speaks your language can create a buffer that will undermine any attempt to see if you make the grade in a foreign language. If immersion in the language and culture is your goal, force yourself to operate and communicate without the safety net of a (paid) native speaker at your side. (One caveat here: Practicing a language one-on-one with a dedicated tour guide who is willing to speak to you entirely in his or her native language may be better -- and more affordable -- than a language school.)