I only remember one my life hearing a sermon preached on this particular parable of the two sons – one who said he wouldn't help but then did; the other men who said he would but then didn't. A student minister (now called an intern) use this as his text once in the little church I attended as a child.
I remember being puzzled. I couldn't understand the two boys and their relationship with their father. For one thing I couldn't imagine a son (or daughter) saying 'no'if a parent asked them to do something. I wouldn't have dared to refuse – I would say yes and I would do it – perhaps sloppily, perhaps reluctantly, a poor job but what he asked for. So I couldn't relate to a son who refused the father's request and walked away. Not understanding how he could refuse his father, I find it difficult to accept that he could redeem himself in his father's eyes by going and doing the job later. The second son is easier to understand in that he said 'yes' to his father. He may have had good intentions, or he may simply have been afraid to refuse, but then he got distracted and left to do his own thing. Good intentions alone didn't get the harvest in. So in my eyes neither of these sons deserve praise for doing the will of the father. The right thing to do would have been to say yes, and then gladly go and do the best job they could. The only way a son could be considered to be doing the will of the father would be to say 'yes' and then go and do the job.
If I remember correctly (and I couldn't have been any more than 13 years old at the time) the minister's message was that works are sometimes more important than words. Just saying you'll do something, giving lip service, is not as important as actually doing it.
(It's not often a person remembers a particular sermon – especially over 50 years).
Now, looking at that same passage of Scripture, I agree with the minister – all we have sinned and come short of the glory of God, none of us are perfect. But what we do, how we respond to God's love, how we treat one another, matters. It matters more than just being agreeable and giving lip service.