I have both an online banking account with Ally (checking) and a Capital One brick and mortar account (checking and savings). There are not many downsides to having an online bank account and I expect in 15-20 years many of the larger brick and mortar banks will either adopt an online strategy or have a "branch lite" strategy of having limited branches with the majority of their presence online.
Online bank accounts typically offer higher APY's vs. brick and mortar branches - your Ally's, CIT's, Schwab, Barclays, ING Direct (Capital One 360) can offer higher rates since they don't have the overhead costs associated with running a branch (i.e. branch associates, rent, etc.). Also, many don't charge ATM fees or refund ATM fees since they don't have physical locations.
I keep a brick and mortar account because there is still certain checks that I need to cash in physically so I prefer to have a branch location. I also keep a savings account (outside of trading / brokerage and third-party managed accounts) to cope with my own personal paranoia that I won't be able to access my online account one day and will have to resort to going to a branch
There's also upside for having a branch location if you are looking to being a borrower, having a prior history as a depositor with your branch goes a long way if you are looking to take out a loan.