So, basically (if I understand it correctly) we agree on the following:
1. Firaxis intended to make the G2 and G3 promotions accessible for GWs (Gallic warriors), but not for all melee units. (It's very unlikely to *accidentally* arrive at the data in the XML and the coding that checks for the "legality" of a promotion when you intended to allow G2/G3 for macemen too).
2. The civilopedia is confusing in that regard. (Basically, the way Firaxis implemented the guerilla promotion line for GWs is a hack of the system. The way Civ4 is coded, all units of one category have access to the same promotions. Firaxis would have had to make an extra unit category for the GW to implement the different promotion possibilities "legally", but this would have complicated other matters, for example it would have made the shock promotion useless against GWs. So they used this hack instead, which the civilopedia can't really cope with.)
3. We disagree in the question whether it would be better or worse for game balance if the macemen could promote along the guerilla line too (given that they already have G1). Bursk says that this advantage is offset by the other disadvantages the Celts have, ori fears that the guerilla macemen might become unbalancingly strong. Personally I don't have much experience with the Celts, so I can't provide a well-based opinion here. I do however think that the change as proposed by Bursk would basically provide the Celts with a second UU: guerilla macemen. If I'd play the Celts then, I'd probably never build macemen, but keep building GWs (are these even obsoleted by macemen?) and immediately upgrade them to macemen, which could then gain experience until they earn G3. Doesn't sound very balanced to me. Even *if* the Celts have a crappy UB, there must be better ways to address this than to give them easy access to hordes of guerilla macemen.
4. Bhruic, you say that the scope of the change is so small that you're not sure there's any reason not to do it - depending on how which position someone has towards (3), you're probably right - for the unmodded game. However, in a game with Civ4's modability, I'm not sure whether we can estimate the scope of such a change correctly. (Admittedly I might be a bit too careful here.)
5. However, as far as I can see, the actual problem is not that macemen cannot get guerilla promotions, but that the civilopedia gives confusing info due to the hack-like way in which the Celtic UU's special ability has been implemented. The solution I'd prefer is to fix the civilopedia so that the confusing info isn't displayed (fox example, add a "legality check" like the one that prevents macemen from promoting to G2/G3 to the civilopedia display.)
Edit: Just checked the Civilopedia: Interestingly, the entry for macemen does *not* list G2 and G3 as eligible promotions. G2 and G3 do appear for Gallic warriors, but not for the other melee units. So the civilopedia does not simply check "ah, this is a melee unit, so let's list all promotions eligible to melee units", instead it checks for each promotions whether it can actually be attained by this unit.
The only pages where confusing info is presented are the info pages for the G2 and G3 promotions. I think the best fix for this problem is to introduce a check there - i.e. instead of simply mirroring the data in the XML entry for these promotions, the page should only list unit types which can legally get the promotion in question and all promotions that are required for it. Perhaps this can be coded by a loop that checks all unit types, discards any unit type that has at least one unit which cannot reach that promotion, and display the rest.