I always interpreted the negative in Elven SD to represent the fact that they are an emotional people. When an elf is angry - they are VERY angry, when sad, extremely sad etc... more on the extremes.
So when an emotional context occurs they are more likely to be extreme - hense a SD modifier.
I agree that is definitely an aspect of having a low SD. Self disipline has many sides to it. The ability to study for a final when you really want to go play. Saying no to that jelly doughnut when you know that you are starting to gain too much weight. Keeping your cool when someone is yelling at you for not filling their Vicodin early - before it should be filled (ya, personal experience here). Staying awake on guard duty when you have a spent day, and most of the night, on a forced march with a full ruck. Fighting back temptation when you know that something is wrong, but would be so wickedly fun to do.
Let characters play how they may, but there are times that the stats of their PC can greatly influence play. I've been in many games where the physical stats predominate, but the mental stats languish on the wayside. Like a character who wants to bend the bars of his cell to escape - but has -10 strengh mod - no GM that I know would say, "well that's the way they want to roleplay their character so I'll let it happen". No, the GM says, "make a role and hope that the gods of luck are with you on this one". With mental stats, I'll do the same. For example, a smart player with a dumb PC does not get to come up with a brillant idea to swindle the nomadic traders while framing a rival group - no, he has to make role against his reasoning. A PC (with low self discipline) who loves to loot suddenly gets practical and decides to first secure the area... nawww, first make an SD role. Too often GMs let smart players roleplay dumb characters smartly (did that make sense?), or in this case, low SD characters wisely.
This has worked very well in our game and lead to many troublesome, but humorous moments. Since this is the first time the party has played RM (all are former DnDers except my daughter) it has been a good learning experience. They know I am a slightly evil GM who will exploit weakness' in their PCs at some of the worst possible times.
Nders - I guess we just disagree. I play elves (or anyone with low SD) as one who may have many talents (smart, charming, athletic) but they are also very foolish. Low SD can have a real downside - just as any of the other stats. SD is not just there to add a few extra development points. Non of the stats should be used only as a percentage increase/decrease to a role. I believe it much more enjoyable if the stats also help shape the PC. To see how stats can effect the roleplaying of a character it is good to understand the extremes of each stat 01-10 vs 90-100 - from low end to high. Here some examples:
Ag: clumsy, awkward, bull in a china shop -- gracefull, kinestically poetic
Con: Febile, sickly -- vital, tough
SD: Foolish, impulsive -- iron will, focused
Re: Dense, block headed -- clever, very bright
Now if you want elves to be more Tolkenish and the wise elder race of the world - that's fine, I don't. Every world is different.