Blue
Ravenous Bugblatter Beast of Traal
Fifth edition was not designed with multi-classing in mind. The one page of optional rules was clearly included as an afterthought. Whenever I've seen a multi-class character under discussion, it looks like it's exploiting the rules rather than working with them. As such, I choose to not multi-class, even if the option is available in that particular campaign.
I'm not sure this is a supportable statement.
Multiclassing rules came out in the the D&D Next playtest, so it wasn't a "last minute addition". Also, many ideas were absent or were removed for part of the playtest to focus on the parts they were working on.
Moving from Next to 5e, we has have design discussions where Mike Mearls talks about multiclass balance passes for classes and where it is in the design process because that is part of any new class' formal design process. (That's also why they don't suggest multiclassing with UA material - they want to get feedback before they spend resources polishing and that multiclass balance is a later step.)
So we know multiclassing is both not a last second addition and is a specific balance step in their class design process. It looks like it's pretty well integrated.
This isn't saying you should use it - that's up to you and up to your table and we need to respect that. But just because it has roughly the same page count assigned to it as ability score generation does not mean it's hastily considered nor an afterthought.