I did the same thing when I had WM. Exchange is a great product if you need secure email, allowing other people to see your availability, sharepoint, etc. So if you use all that, then it's worth the expense. For me, I don't need that level of sophistication. I just need to sync my calendar and contacts. Gmail sync and outlook connector are surprisingly good.
I don't ever look at my google or live accounts in a Web browser anymore. Everything I need is in Outlook or on my phone.
You can get spam with any email address. There was an email host that got attacked about a month or two ago, and they were the host for many very big companies. I started to get spam in a bunch of my email accounts and still do. I don't know how Exchange would stop that.