I think I'm in agreement with most of the posters in this thread, that these trails can be responsibly and respectfully used in all seasons as long as one uses good judgement and errs on the side of avoiding the use of trails that are likely to be too muddy. We wouldn't be having this debate or reading this thread if we didn't all love to hike in Vermont.
That was a good link, Una_dogger. I think people should also note that the GMNF is in agreement with the GMC in that they both "strongly discourage hiking on the Long Trail System and on other Vermont trails in mud season, particularly at higher elevations". Either way, it makes little difference if a regulatory agency or a hiking club "strongly discourages" something since they both are making a recommendation, not enforcing a rule or law. I think these groups are appealing to the hiker's better judgement, and that the majority of the people on these boards understand why and are likely to heed the official recommendations. It is the casual hiker that might not understand the nature of hiking at this time of year... that stepping around that icky mud puddle or walking in it up to your calves isn't always the best thing to do.
The "concrete examples of trail dates" are general guidelines that are usually suitable for hiking. This means that if we have a mild winter or a dry April, you could get away with hiking earlier. Usually suitable does not mean hiking in the fragile alpine zone on top of Camel's Hump on May 29th would be a good idea if it had rained hard the previous weekend. It is subjective, and their opinion of a muddy and unstable trail might not look too bad to me or you.
I don't think the GMC or the GMNF want people widening trails OR churning up mud. In my humble opinion, it is better to avoid trails when there is a high chance you'll end up eroding them. If I were to start hiking, I probably wouldn't want to turn back if I reached a swampy area 300 vertical feet from the summit.
Either way, I'm not going to tell you not to hike during mud season since I have been out there hiking in selected areas myself - that would make me a hypocrite. But I will try to encourage responsible use and knowledge of the reasons behind their recommendations, and I can legitimately say that I am aware of the situation and I tread with care on the trails and at the times when I feel I can do so appropriately.