There is a huge amount of reasons for someone to encrypt their communications over the Internet. For me, encrypted communications are not optional, but plain communications are an alternative I am forced to use in some cases, such as messages sent and received on wikis or mailing lists, or because the other person lacks of needed technical knowledge to encrypt messages (something I try to avoid by teaching others).
And now that you are here, y encourage you to use encrypted communication with me whenever possible.
By using PGP you can sign and/or encrypt the email you send me, and I can do the same when I reply, no matter if you use GNU/Linux, OS X, or Microsoft Windows. In anycase, I recommend you to manage your email with Mozilla Thunderbird and Enigmail, in order to encrypt your messages more easily.
You can find my PGP key on key servers. My address is email@example.com, and my fingerprint is B387 F3B1 0F2C F46B 36AD FAFF 7BC3 594D F7C0 E1A3.
Email service trust
If your are unable or unwilling to encrypt the email messages you send me, but you wish a little more trustable communication (for instance, it is very likely that every communication made by most of the services under the US, EU, China, and other nations' jurisdictions are permanently saved on government databases), I encourage you to use only the services listed on Privacy-Conscious Email Services, because these guarantee socially and legally the privacy of their servers content.
The only protocol I use for IM is XMPP, and I refuse to use any other. We can also encypt our communication by this XMPP, using OTR. My address is firstname.lastname@example.org, and my fingerprint is E95CB6E6 22751983 CA8F3F67 3DFACBFF 0FA3A1BC.
On freenode, mi user name is ralgis, my cloak is @gateway/tor-sasl/siglar.