Cheap temples and cathedrals just means that you have to start paying upkeep quicker. If you need happiness, go grab some luxes, but building happiness buildings is a total waste 90% of the time. The quick change is great, until you think about what it costs you. Are those 5 or so turns worth what you give up from not having a different trait? I'd rather lose 5 turns than give up a trait.
If YOU play Conquest/Domination/AW on small-normal maps, place your cities C-X-X-C and think of temples as three spearmen or two swordsmen not built, then you do have a point! However, on large-huge maps with cities C-X-X-X-C or C-X-X-X-X-C and different victory conditions, there are serious reservations.
First, during the first 3000 years or so there won't be more than a couple of luxes within reach. At least one of those will cost you >30 worker turns plus the need to expand a worker as a colony (another 350 or so lost worker turns) and defend it. That translates as a hill and a BG not roaded and mined. This loss in shields and gold is far greater than the gold one pays in upkeep as you only have half a dozen or so temples to pay for!
Later in the game, esp if you play large-huge maps, a lux is cheaper than upkeep for 40-odd temples and cathedrals - but by then with metros you can do with - and afford - both!.
Second, IF you don't build temples, grab nearby luxes and fortify troops, you will have to sacrifice science and gold for lux on the slider - especially if you want to have a shot at building wonders. Now your advice may be fine for AW and Conquest/Domination on the smaller size maps, possibly for higher levels. For Cultural, Wonder, Diplo or Space Race, your advice is advice how to lose such a game.
And I didn't even mention that those turns saved changing government (about 4 the first time, eight the second) equal three techs researched - in effect equalling the free tech at each age for Scientific civs - huge amounts of gold and shields produced. Ooops! I
did mention it!
