Depends what you want to do....
If you want a fighter with some magic, it's easy and better to take a level in cleric while at the low levels, then keep advancing as a fighter/barbarian/whatever. Granted you only get a few heal spells and a limited ability at turning, but it's much better than nothing. If you take a level in arcane, you are restricted by the armor in most cases... so it becomes more of a wasted level unless you are wearing light armor, i.e. rogues.
If you are aiming at a strong caster, it is a big advantage to take 1 level in fighter or rogue in order to get the extra skill slots and proficiencies. Not only that, but having 1 level in fighter early helps with the HPs and not getting KOed all the time in the lower levels. And with a level or two or even three of rogue gets you all sorts of abilities that are a bonus to have through the levels - backstab, uncanny dodge, etc.
Granted, multiclassing makes for a slightly weaker PC than a 'pure' class, but in adventuring it is always good to be multifaceted in case the main expert at something is out of commission. I think it is balanced pretty well, as a jack-of-all-trades is gonna be weaker than a character that has focused in one class.