I'll make a case for trailblazer 1&2, though I generally agree with the concerns raised in this thread...
Trailblazer scouts make great surprise-pillagers, to take out key resources, or well-placed but unmanned citadels (an AI favorite)... I definitely find them *useful*, and at least in my play-style take these as often as survivalism, though admittedly I rarely take it all the way to TB3, nor any of the other XP promos. TB2 is much more a niche choice than TB1, but for the pillage-then-escape role, it can be invaluable with just 1-2 of the applicable tiles around.
Maybe its just me, but I rarely/never see any AI "scouts" (unless from early ruin upgrade) or "explorers"... zeppelins too for that matter. I have seen a few TB AI recon, but they usually die quite quickly.
In other recon unit matters, I dislike how losing an early unit usually determines whether you have a super-elite paratrooper ~3000 years later. I suppose this is the case with other units to some extent, but w/o a pure combat role, adding xp to new recon units at a similar pace as is available early-game, is nearly impossible. I truly wonder if any of you actually build new recon units from mid-game onwards? at the very most i build 1-2 early to help my original pathfinder explore, and then none til paratrooper. I wonder if this role could not be improved more generally, beyond XP promo issues.
Out of the various fixes here, I like the idea of making the XP promos free to later recon units... but I would add the qualifier of only adding these to newly built units, and not upgraded (i'm pretty sure there's a 'notwithupgrade' field in existing VP promo tables.. not to be confused with 'lostwithupgrade'). There is less exploration recon xp at this point in game, new recon units especially need another XP mechanism available to increase their viability