This is nonsense. No browser cache, no file download, no kind of database, no development, no compiling, no updates and upgrades, no ... the list will be endless. Most programs won't work at all. You will end up with a secure system that can do almost nothing.
Which program will not work? I'm thinking and I can't find one. I use that technic a lot and didn't find anything not working.
Don't forget we are talking about putting the SD card in read only mode. Not the full system! some temporary folder will have to be in read/write mode using other device or ram disk.
Also why I will put a development system in read/only mode. This is on final product stuff not on a unit you use to design.
B.T.W. The little script just reroute the /var/log to a ramdisk. The SD card is still in r/w mode.
On my previous setup I was transferring temperature log and image capture from mjpg_streamer and the system was crashing every day. I had to re-do the card often because fsck was not working all the times. so I decide to limit the writing on the card by rerouting the /var/log to a ramdisk. And I also transfer the temperature log and the image capture into a temp folder which was also a ramdisk. A script was then use to transfer the data into a network drive ,my cloud from western digital.
I'm using lighttpd with dynamic web page using Highcharts. So the webpage location are in a ramdisk folder. I do have the webpage skeleton into the sdcard and they are use has backbone only.
Since I did that , I never had to re-do the SD card image.
So if this a good idea? yes . If it is needed ? No for the vast majority of the times.
Well I do have some industrial system using read only root fs. It doesn't mean that everything is static. You could mount r/w stuff on it . usb memory stick , external drive and for me it is network mount.