To the first, I would say that since Scripture is so clear elsewhere that true believers cannot lose their salvation and that those who do leave the faith demonstrate that they were never a part of it (1 John 2:19), I don't think any of the passages that warn against falling away actually imply that people can fall away. With regards to probably the most difficult passage of this sort, Hebrews 6:4-6, I understand it as referring to those who are open members of the church for a time, being blessed by the other believers, maybe even blessing other people, but turn away (Judas would be a good example), and not to true believers. And even the author of Hebrews dismisses the possibility that this could happen to true believers by his words at the end in Hebrews 6:9
9Though we speak in this way, yet in your case, beloved, we feel sure of better things--things that belong to salvation.So I would say that the warnings in Scripture exist as warnings and not as statements of truth. We are warned by Scripture, and that's one of the means by which God preserves us.
Regarding the "license to sin" objection, I will admit that I know of people who, thinking to understand the doctrine, use it as that excuse to not care about their sin. Personally, I don't think they should have any assurance that they're saved at all. That they are elect may be the case, but I don't think that they can rest upon those promises as applying to them in this case since that's exactly what the warnings of Scripture are about, to test us and to grant as assurance and remove it when we don't bear fruit in keeping with repentance. One who is saved cannot keep on sinning.
So it may be the case that people will use it as an excuse to sin, but that's not enough to dissuade me from believing it, as Paul faced those very same charges when he proclaimed justification by faith in Romans 6:1, and I think justification by faith is closely linked by Preservation of the Saints (are we saved by faith or by persevering?), so I'm ok with being lumped in with Paul on this point. =D