this topic should not become a "best groupware" war, anyway, my 2€c:
Zarafa won't become the default webmail application in SME because, essentially, it's not full OOSS.. and, please remember, it must use its own mail daemons.. finally, since Zarafa stores everything in a biiiiiig mysql database (mine was about 4 GB), it could be difficult to backup and restore (think about the pre-backup event, that makes mysql database dumps)
SOGo is already available as a contrib (thanks to Nethesis crew), is totally OOSS, uses standard protocols, use every kind of mail daemons (smtp, imap etc), uses a standard email client (thunderbird) available on all OSes out there.. should us be able to install samba4 on SME9, we could get it work as a FULL exchange replacement (full integration with outlook without any kind of plugin or connector and totally for free)
EGW is "only" a web application, and in some cases (all my customers, for example) won't fit people needs.. in a country (italy) where broadband connection is sometime unavailable, webmail is not a good choice..