Re: your software is undemanding or can't utilise 8 cores.
It's not just parallelisable applications but how many processes you want to run. I'm used to running many cpu-intensive programs at the same time whilst doing lots of other stuff whilst they get on with it.
"Outside rendering, are there many other widely used parallelisable applications?"
Plenty of scientific ones, but even for my usage when travelling I use scripts to further process sets of directories of jpgs(that's 100s of files in each, 5k*3k ) (scaling/sharpening) to produce sets that match the native resolution of various devices they are going to be viewed on. Having 4-8 copies of that code running can give a laptop a good work-out. I also do the same with video using ffmpeg to convert 1080/50p to 720p again for display purposes, and during this time I still want to write, email, upload, browse, and process RAW DSLR images, build panoramas etc. That's why I bought an i7 with 8GB. I could have done the rest with my old Celeron 1GB laptop ( I admit the screen hinge is glued with Araldite and possibly living on borrowed time), but when 1 minute of video needed 30 mins of rendering it was too slow.