I don't beleive there should be a penalty for leaving STF's early.
I just got on 3 STF with noobs who kept destroying the gateways instead of doing 10 percent. And instead of staying in there for an hour or more dying and damaging my ship. I would like the opportunity to leave and try again with hopfully a more experienced group.
I also hate it when you try to explain the 10 perent rule and noone reads the chat and destroys the gateways making it harder still.
We should be able to leave an STF with no penalty as long as your in it for about 5-10 minutes.
You should not do an STF in a PUG unless you are willing to stick it out to the end, which means that there has to be some kind of penalty for leaving.
Now, once in a while you do get into a hopeless situation where you just are not going to win with the people you have. In that case, you just have to bite the bullet and take the penalty. I have only had to do it twice in nearly 200 STFs. Once on Infected elite and once on infected space. That is just life. If you do not like it, you can always go with a pre-made team.
Imagine a group with 5 people. One is a noob and destroys a generator early. After that you get annoyed and leave. Buy doing that you're just as annoying as the noob who ruined the Optional in the first place!!! The other 3 players who did perfectly well have to end the elite STF with 4 people only. Or if somebody else joins, he has to end a game with an already lost Optional.
So NO! Leaving early should have exactly the penalty it has at the moment, if not even a more severe one.
When I join STF's I'm in it until it's finished. I go in EXPECTING not to get the optional, hell with elites I still get the 2 EDC at the end and quite frequently a prototype salvage so it's not a big of a deal as your making out. Yeah your going to get people who don't always read the chat or sometimes they might make an honest mistake! That's gaming for you, if you go into a PUG expecting them always to be perfectly organised and get a good run then your very much mistaken.