Champions split the xp up when together, and that may discourage you from massing them up, but here are a few points:
1. Loot can only be used by champions and henchmen, so while you have as much standard equipment to create custom units with as you have metal and crystal, there are some fancy items which far surpass anything that you can make, and only champs can use em. I find it also a lot easier to tank up a champion than a trained unit starting from the early game. Many enchantments are champion only also.
2. Champions share experience, but if by using more than one champion in an army allows you to fight and win more battles, then that is better than putting off a fight and its rewards till later. Having 2 champions fight in two separate but equal battles is the same overall experience for each champion as if you fought both battles with one champion each, and trainer gives you more experience if you join them up.
3. By saving time that could be spent on long term research and building infrastructure, you can get "ahead" if champions do a lot of the heavy lifting. You will probably be caught "with your pants down" if you totally neglect the training of and researching of unit techs, because numbers will overwhelm.
I often have at least 1 champion go path of the governor early now to offset the population lost by creating pioneers, unless I get a really good combat champion or some sweet items early. I try to get my sovereign some fast early levels to raise prestige as well. If my strategy does not involve creating some early units, or I haven't found some nice quest reward units or have summons, I will usually stack up my champions so that I can keep taking out monsters, quests, and lairs or fend off early unexpected wars.
Also the map size will affect your strategy. On smaller maps I can use champions to win an early war and not even worry about getting deep in the magic or war trees to make some uber armies. I think that the longer the game, the more you will be forced to use and focus on units. You will not be able to be everywhere at once with your champion stack on a large map, so you might as well get some beefy units in there and split them up.
The suggestions were just to create more integration between your civilization and your army and one thing right now is that it is viable to use champions together even if the best strategy involves diverging and splitting them up in circumstances. You can rush opponents (factions and mobs) with champions and with some luck on loot you can ramp up your champions power quickly and independently of any empire building going on, especially if you are an adventurer (Altar), and then you don't even have to raise the gildar.