For example it seems like anti-virus, anti-spam and firewalls have sort of merged into one product. Is there a downside to this? I don't know of any software firewalls that can be purchased without an AV, is there a reason this has happened?
Here are some to think about, mainly from the "home user" perspective. The enterprise environment is unlikely to use a single vendor for all security needs. If they do, the setup and support package usually eliminates all common mistakes.
For me, a big downside is bloat. For a while I used Comodo firewall on my windows home machines, but gradually updates started including some stuff that really should have been optional, like live chatting and such.
Also, over time, I'd accumulated a large amount of images to make up the gui that weren't really necessary.