Why should an e-mail app be any different?
People don't use most of the features in most of the programs they run. For that matter, they don't use most features in other things. How many bothered to learn how to program their VCRs? What percentage of features on smartphones or GPSs do they use? Heck, i just bought a new ratcheting screwdriver that came with 12 or 16 bits; I'll probably never use half of them.
The number of available features is a factor in many purchasing decisions, regardless of whether they'll be used.