The issue of spell casting focus changed considerably over the period of the play test, from allowing you to add your proficiency bonus to attack spells to the (mostly watered-down) result of it being a substitute for a component pouch.
When we discussed this in August, I pulled to gather the evolution of my concerns with the implementation in
this post here.
In terms of flavour, the focus is a GREAT idea -- it's fun to play with, and the image of the paladin or cleric holding up a holy symbol, the wizard with a staff or crystal ball, the bard and his instrument, or whatever is iconic. And the flavour is there regardless of the mechanical effect.
The problem comes in implementation, because it just is never going to be balanced between classes -- there's always so much more that needs to be in your hands. And so you start to get fudges:
* the cleric and paladin can wear their focus around their neck or affixed to their shelf (but then what of somatic components?)
* the bard who plays a one-handed drum can wield a shield but the flautist cannot
* the wizard's staff doubles as an effective melee weapon, but the wand does not
etc.
* problems interacting with tool proficiencies: what does it matter if the bard is proficient or not with the instrument, when spell casting is independent of any proficiency bonus?
So -- if you assume that these things should be balanced across classes (and the general feel of 5e is that that's a design goal, cf. spell casting slots, etc.), then you have options:
a. free for all: weapons can be stored and retrieved without any expenditure of resources; when the cleric needs a symbol it can be used freely. Such a rule encourages other behaviours (such as fighters carring golf bags of weapons, etc.), which some will see as undesirable. (EDIT: the wii-strap solution is essentially a workaround of this type)
b. reduced mechanical effect: foci do very little, and are essentially just decorative. This is what has been chosen (though we see from Mearls's tweets that there's some of a. as well), with the focus being no different than a materials pouch.
c. insist on strict effects: a focus is a weapon that requires proficiency and must be stowed or wielded, etc. to have its effect. This was my preference (because it requires players to choose their actions in game, accepting consequences for a desired benefit), but (as has been clear throughout the play test) I was in a minority on this.
The result, though, is that we have inconsistencies still (e.g. the druid or wizard staff), and the benefit of using a focus is negligible.
I find it hard to believe that anyone really likes the final implementation of the whole focus issue (including the interaction with tool proficiencies, etc.) as it is, but what we have is a safe and relatively unchallenging
rule that allows flavourful play without taxing players in any real way, and which has some options that are mechanically better for some users than others.
