I think the standard thought is balance the two and try to make their numbers match. One doesn't out damage the other and so on.
Multi class offers so many more options, playstyles and combinations. Multi allows you to combine so many more skills and spells for a huge variety of play styles. Even if mathematically a single class druid matches a druid/rogue in terms of damage, survivability etc that the variety and options multi classing offers count for something.
Also, multi classing offers much more regarding role playing than just simple damage and healing effectiveness. If I'm serious role player, multi class is guaranteed to closely match how I envision my char than single class. This is because combining two different classes can arrive at the combination of skills, spells and flavor that a player envisions. Single classing is much more limited.
I admit the less a person cares about role playing the more single class might suffice. But the more a person wants to create a personality and philosophy for their character then i definelty claim that multi will come much closer to the desired match. The more specifics a player adds to their character the more multi classes can come closer to the mark because of the variety and amount of skills.
In short, I think numbers and math are not the only thing to consider when it come to balance. Even if multi and single class are on even footing regarding damage, heal, penetration, recovery time etc, multi class still hold advantages over single class. Because of this, single class should receive some other things to make more desirable. A simple example might be subclasses only for single class selections. Unique histories and cultures for singles class. Video games are not only about math when they are labeled rpgs.
Edited by asnjas, 11 August 2018 - 01:51 PM.