Generally-speaking, the Ftr 19/Wiz 1 will have more productive things to do with her actions (of all varieties) than spellcasting. As has been mentioned before, things like Shield and Expeditious retreat are most likely to come into play when the character has already effectively lost and is looking to escape with her life.
I disagree. If you have access to spells like Shield and Expeditious Retreat, they can of course be used to make escape easier. However, that's a waste in my opinion (at that point you're basically going into an encounter expecting to lose).
Shield is an amazing spell for a fighter, given their already advantageous AC. Let's assume that the Fighter 20 and the Fighter 19/Wizard 1 are facing off and both have a Strength of 20 and that their magic bonuses for weapons and armor cancel out. Assuming Plate armor (AC 18), they both have a +11 to hit, giving them a 70% chance to hit each other. Now the F/W wizard casts shield. Suddenly, the F's chance of hitting for that round drops to 45%. The he F gets 4 attacks at 45%, versus the F/W's 3 attacks at 70%. Now the F has a choice to make. Does he use action surge now and take his extra attacks at the reduced hit rate, or wait for a round when shield is not active? The F/W, meanwhile, can go nuts on the F with action surge. I don't see how anyone can consider that anything less than a decisive advantage in combat. Granted, this means that the F/W isn't going to be able to make an OA this round, but the odds are he wouldn't need to anyway.
It's the same with expeditious retreat. If an encounter starts beyond their base speed, the F has to either Dash (which means he can't use that action to attack) or ready an action and wait for the enemy to come to him (meaning he only gets one attack). The F/W can use his bonus action to cast Expeditious Retreat, potentially allowing him to close the distance and make his full 3 attacks. He'll lose an attack if he's a TWF, but otherwise what does he need his bonus action for? Being able to make 3 attacks in the first round is clearly superior to making 0 or 1 attack.
Clearly, these spells have significant advantages beyond that of just retreating.
While I agree that Disguise Self and Comprehend Languages can be useful, they also seem like things that simply get the character the ability to participate in a social encounter. And, let's face it, the player should already have some means of participating without those spells.
It's not a matter of being unable to participate. It's a matter of having the ability to approach the situation from a completely new angle.
Let's say that the fighters are trying to get some information from a butler. F20 might try bribing the butler or intimidating him into giving up the information, and there's nothing wrong with that. F19/W1 can do those things and could also use Disguise Self to appear as someone to whom the butler would willingly give the information. Of course, this approach could fail (FW says something that gives him away), but if it succeeds it has the added advantage that the butler doesn't know that the fighter knows. Clearly advantageous.
I will grant you that feather fall is handy, but there are several ways to avoid a fall to begin with that are all well within the reach of mundane capabilities.
The FW is just as good as the F at utilizing those mundane capabilities, but unlike the F he has an additional trump card in Feather Fall for when they fail. Clearly advantageous.
It's also worth pointing out that power and breadth of ability are hard to consider because of the differing nature of people's campaigns. In a dungeon crawl campaign, the social pillar can be virtually non-existent. Likewise, in the courtly intrigue campaign that I ran the exploration pillar almost exclusively consisted of hiding and sneaking so you could overhear plots and various nefarious intentions.
Completely true. But it's true of many character choices. If you're running a hack and slash dungeon crawl, an Evoker is probably going to be notably superior to an Enchanter. The opposite is true in a combat light, heavily politicized/social campaign. In the latter case, the majority of fighter abilities will rarely see any use at all.