I won't try to speak for others who have argued for years that the most likely scenario for who Jon's parents are is that they are Lyanna Stark and Rhaegar Targaryen, but I know I have spent many, many hours since I first came to this site in 2007 arguing just this point. If you had read the 33 versions of this thread previous to this one you would know that, but perhaps that is too much to ask of new people to this site. What isn't too much to ask is that people don't make categorical insulting remarks about people they obviously don't know anything about.
On numerous occasions I have pointed out that the variation of "Aegon at the TOJ" I find most plausible - wherein it is Rhaegar, not Varys, who is responsible for the baby switch Young Griff tells us about in ADwD - and how it has little to no direct evidence to show it happened. What evidence it does have, the actions of the Kingsguard trio at the Tower of Joy for instance, is better explained by a legitimate Jon's presence there than Aegon's. Why? Because we have numerous clues pointing to Lyanna and Rhaegar being lovers, the manner of Lyanna's death hinting at childbirth complications, and a timeline that fits amazingly with Jon's conception and the window for his birth. Any of the Aegon at the Tower of Joy scenarios lack this pile up of clues to support it.
That doesn't mean this theory has no value. Part of what I find of value is a process of sorting the wheat from the chaff. An example of this is the exercise I did earlier with just using references in the text to show that it is not possible for Jon to have been conceived after the events at the Tower of Joy when Ned goes to Starfall. It is helpful to understand what is not possible in order to consider what is possible.
So when people raise legitimate questions about some of the variations of Aegon at the Tower and the theory is refined to deal with those objections, or we can throw out variations as impossible, it helps us all in considering what is possible and what is likely. It is a process that shouldn't be dismissed.
Moreover, I think it misses a huge, huge development with the publication of A Dance with Dragons. For the first time we have a character who claims to be Rhaegar's hitherto assumed dead heir. Whatever one thinks of that claim it is stupid to ignore its implications on many aspects of this story. One of which is that a baby-switched Aegon at the Tower of Joy would explain the Kingsguard trio actions in much the same way the legitimate Jon as the son Lyanna and Rhaegar does. It needs to be explored, not shouted down.
Having made these points before, let me introduce the "but." Or perhaps I should say "buts." Aegon at the ToJ is not a competing argument to R+L=J. It is an addition to, or can be, to R+L=J. As such, to place the straw man argument that all people who support the one oppose the other is clearly false.
Moreover, the implication that this is some kind debating war over who supports R+L=J and who doesn't misses the entire point of these threads. It seems to me this should be a place to disscus Martin's work and collectively shed some light on them. Debating wars only shut people up who grow tired of reading them. They win nothing. I have no problem with occasionally heated arguments by people who strongly believe something. I have little patience with people only seek to prove themselves a superior debater. There are other places for those kind of exercises, this shouldn't be one of them.