Any downside to using multiple mail and calendar apps on iOS?
Yes, you definitely will double the data sent, if one uses two apps for the same account.

However, using different programs may be useful.  For example, if you want to separate your professional Exchange E-mail from your personal accounts.  This way, there is a logical separation, and you run less risk of accidentally sending out a personal mail on a business acount, and vice versa.  Some mail apps even have password or PIN protection (for example, Divide), which will keep your E-mail accounts safe, even if someone swipes your phone when you are using it.