I beg to differ. While most modem had enough juice to handle enough packet to saturate the pipe supplied to your home, same cannot be said for other router-handled functionality (i.e. firewalling, etc etc.)
As for the modem/wireless access point thingy, the thing you should consider is the stability of software and capability of hardware. There are in essence no more 'pure adsl2+ modem' nowadays as router function is cheap and is often integrated on the chipset anyways, and the only thing distinguishing the two is the presence of ethernet hub, and perhaps wifi radio.
You can most likely change the router firmware into something like dd-wrt, tomato etc. and these allows for more sophisticated routing to be done, and (+/-stateful) firewall to be set. same cannot be done for most modems, and you can choose with price, for processing power of the router.
The point here is ability to customize the firmware.