The problem there is that in singleplayer, the AI will happily get its fleet killed repeatedly, and this lets you easily level up your capital ships.  In multiplayer, you will very rarely have multiple level 6 capital ships.  I have never seen anyone manage to get a level 6+ Dunov and Kol simultaneously in multiplayer.  They're just too weak at the lower levels so people rarely get them in the first place, and even when you do get both (I often do later in the game) it's rare that either will reach level 6.  I've never once seen both reach level 6 before the end of the game (or, more commonly, one dies prematurely).

I do think we should go overboard here.  If you manage to get multiple high level capital ships, you deserve to have something amazing that strikes fear into the hearts of enemies.  If you do the math, a Kol spamming level 3 gauss cannon non-stop is really only dealing as much damage as 6 or 7 Kodiaks.  The difference is, those Kodiaks didn't require you to painstakingly level up a Kol, and they don't have antimatter restrictions either.  A high level Kol is basically irreplacable; if you lose it, you may never manage to get another!  The Kodiaks are a dime a dozen; if you have a solid mid-game economy you can afford to replace a few dead heavies.