Attorneys have a duty to keep clients informed about what is happening in their case.  Email is a fantastic tool to communicate with clients.  More complicated issues can be presented and outlined.  Graphic images can be incorporated.  Documents can be attached. All without the hassle of playing "phone tag" or finding out that your client's voicemail is either full or has never been set up.  

Email is at least as secure as sending a letter in the mail, and a great deal faster and less expensive.

But, it is not something that needs to be short cut.  Attorneys should not cc or bc clients on emails to opposing counsel or the court.  At least one bar association that has looked at the issue has found that doing so gives consent to the other lawyer contacting your client directly, at least as far as opposing counsel hitting reply all.  It also makes it too easy for the client to send the opposing counsel direct mail by hitting reply all.  

An attorney should take steps to prevent direct communication between the client and opposing counsel.

The best ways to accomplish that are (1) to forward the communication between attorneys that you want the client to be aware of and strip out the header information that includes the email addresses or (2) to compose a new email and cut-and-paste the relevant part of the email conversation into that email.  Either way means that the client won't inadvertently send an email to opposing counsel and that opposing counsel won't have consent to contact your client directly.

Proper use of email implicates a lawyer's duties of competence and confidentiality.