I'm not too sure how helpful this would be. If I didn't get off at the last exit, then it's probably because that exit wasn't the one for my destination. It is much more helpful to have periodic signs that tell you exactly how far it is to the next exit. I already have some idea how long ago I passed the last exit anyway.

On a recently completed section of highway in this area the DOT installed milage markers every 2/10ths of a mile and labled them accordingly. This might have been a money saving move to eliminate the old 26 reflector markers per mile or an informative item so stranded motorists could pinpoint their location when calling for help. Either way, it would be no big deal to add a small "NE" and "LE" (Next Exit / Last Exit) to the bottom of the sign as well.

I'm thinking though that cell phones have pretty much eliminated the need to walk along freeways to get assistance.