They're still worth healing, its just that not many people see that. I for one, am a damn good scout. I always make more than my fair share of kills. Thanks to scout's speed, you can take out anyone if you're good enough.
Yeah, it's not all just general aim and swinging your bat around. but my point is- since scout's health is so low, it would be unwise to heal a random scout instead of say, an engineer building something or a demo setting traps. there are very few instances in which i'll heal a scout as medic simply due to the fact that there are other classes who stay alive much longer and i won't end up wasting heals.
I personally am less likely to waste much of my heals on a Scout that is going to be killed in no time. That and I'm not going to chase the little fucker around. They have to come to me. But I mean, I'm a biased medic when it comes down to it. There are certain classes I see as being higher priority. Then again, it really all comes down to the game and what's happening at the time, so it's all relative.