just commenting cause I found this by googling for an answer, and actually I achieved exactly what the OP wants, with Tasker and Secure settings. No caveats here, working fine. Using it both on a Nexus 4 and N7 on CM 10.1.
First of all, have the slide to unlock enabled, then enable your pattern. When Secure Settings disables pattern unlock, it goes back to what the phone was set to before.
Profile: display off (7)
Event: Display Off
Enter: Slide/pattern (12)
Abort Existing Task
A1: Secure Settings [ Configuration:Pattern Lock Disabled/Wait For Unlock Package:com.intangibleobject.securesettings.plugin Name:Secure Settings Continue Task After Error:On ]
A2: Wait [ MS:0 Seconds:0 Minutes:5 Hours:0 Days:0 ]
A3: Secure Settings [ Configuration:Pattern Lock Enabled Package:com.intangibleobject.securesettings.plugin Name:Secure Settings Continue Task After Error:On ] If [ %SCREEN ~ off ]
A4: Wait [ MS:0 Seconds:1 Minutes:0 Hours:0 Days:0 ]
A5: Secure Settings [ Configuration:Pattern Lock Disabled/Wait For Unlock Package:com.intangibleobject.securesettings.plugin Name:Secure Settings Continue Task After Error:On ] If [ %SCREEN ~ off ]
The "continue task after error" setting is probably not required, I was just testing and left it there. Two key settings are on Tasker, disable beginner mode on settings so you get the task properties icon, then there, change collision handling to "abort existing task". In Secure Settings you need to install and enable the helper or the pattern unlock will work but may be unstable (it was on my N4).
Note that in A3 and A5 I enable pattern and disable it again. This is done because if you just enable it, it will ask you for pattern twice in a row, cause you set in Secure Settings' pattern unlock to ask you for pattern the first time. If you don't set it to ask you for it, then the pattern can be bypassed by simply turning off the screen twice.