I've Dm'ed an epic game with cohorts, and simply put there is no real advantage to waste an epic feat on Epic Leadership if you already had your cohort at non-epic levels.
Even when epic, the cohort will advance fast enough and eventually reach the -2 levels limit.
Really, it's only useful if you crate a new character of level 30+ and want the cohort at level -2 right away. Even then. If your cohort is, say, a cleric, at level 17 he will be quite useful for an epic party of any level.
His survival potential is something different however... but 75% of the time cohort really are just support (buffing, skills, healing...). Rare is the encounter that will ignore the main hacking PC to concentrate on the little dude with the holy symbol hanging back there.
Area spells are something else of course, which is why my PC's have taken the habit of telling the cohorts to follow them at least 20 feet behind.
Too much rambling...