1.)  Because some people think if you get the extra creeps one at a time, you are giving your opponent extra experience.  So some people like to wait till war rank 8 and buy all the way up to catapults.  Its dependant on the game I suppose.

2.)  You'll always find teammates who ignore citadel upgrades entirely.  They would rather focus on buying armor for themselves.   Usually they play regulus, and have losing records.  For upgrades you want to get (as a general rule), I would lean toward FS1 and Currency 1, and obviously the creeps waves as war rank increases.

3.)  I dont care for the experience upgrade.  Its 10% I think, which means when they are level 10, you'll be level 11.  Wow...Personally, I think there are better ways to spend your money.

Honestly, you should only get priests early only if you are stomping the other team. A standard creep wave (5 minotaurs + 2 archers) gives 231 XP. Adding in priests brings the XP per creep wave to 291. This is a 26% gain in experience. It's like buying the second XP citadel upgrade for the other team. I've seen experience players do this, but it doesn't make any sense to me.

There can be good reasons and bad reasons for this. Some DGs require a lot of gold to be effective. Others, like UB, only need 6k worth in gear. DGs that don't require expensive gear need to buy more citadel upgrades than those that do.

Always get XP 1 & 2. By level 10, your entire team will be a full level ahead of the other team, and you will be able to push your opponents off flags much more easily. Beware, however, that you'll be a little bit squishy early on. However, this is acceptable since your towers can provide decent protection at low levels.

Actually, the upgrade gives an additional 10% experience instead of a 10% level bonus. A 10% experience bonus doesn't translate into one more level at level 10. Buying XP 1 will mean that you will reach each level before the other team, but you will never be a full level ahead. In order to be a full level ahead, you need to buy XP 2. It's worth it because it provides your entire team with a level advantage. Anytime you get a kill, you should invest it in the XP upgrade. It permanently sets back the other team. If the other team buys priests early on, you will be earning 46% more XP than them if you buy XP 1 & 2.

I've been playing games recently where XPI or XP2 is bought and I was surprised that it does make a significance difference in levelling.

Wow Catapults are actually a huge gold bonus to the opposing team if they clamp down fast on them. Of course I dont think people would complain about their allies buying cata upgrades >>

I think the citadel upgrades need a slight gold reduction though

The whole team mates ignoring the citadel annoys me

Just played a game with some guy who called me a noob for asking him to buy Currency I on 3v3 Cataract.
He also abandoned me 2 times while we were up 2v1 ...

I'm speaking about valuable upgrades of course. Currency 1, Repair 1, XP 1 and creeps mainly.

So by your argument, assuming all other factors are equal (flag capping, killing creeps), for you to get to level 11 when I reach level 10 you have to buy TWO upgrades in xp?  And thats a good thing how?  That makes xp upgrades seem even more worthless to me.  Levels are not THAT important.  I'll gladly sacrifice a 1 level differential to my opponents to be in a better position money-wise to purchase creep upgrades, cap locks, armor, whatever at the end game because you spent not one purchase, but two on xp bonuses.

                                        Level       XP Req     XP to next    XP to next level with 10% advantage XP to next level with 30% advantage (row 8+ is to 2 levels ahead, row 15+ is to 3 levels ahead)