Friends is good for problems that can't be solved by killing people. For example, if you need to get through a gate, and the guard knows the password, killing him won't get you that password. Casting Friends on him, persuading him to tell you the password, and then killing him is a much better plan.
"But you could just force him to tell you the password!" Well, possibly. Loyalty comes into play here, and it's important to remember that the DM has every motivation to make his guards fanatically loyal.
In general, Friends is primarily useful to help you get information. Yes, you'll have to deal with the person afterwards, but you would have had to do that if you'd gotten the information by threatening to kill them. This way, they should give you correct information; getting information by threatening them is a great way to get information that leads to your death.
A secondary use is getting people to do things they wouldn't normally do. This sounds obvious, but (to stay consistent), imagine we have another gate, only this one has a guard behind it. He won't open the gate for you normally, but with the help of Friends you might be able to talk him into it. Obviously, once you're inside you'll probably have to kill him.
This is not a problem for most effects. Most of them last “until the end of your next turn”, which for effects that benefit something you want to do on your next turn is plenty long enough.
The only case where that produces counterintuitive results is when an effect disadvantages your opponent, as in your defensive spell example or a monk's Stunning Strike, because if your next turn comes before they next act then the effect is “wasted”.
This second class of effects is easy enough to identify on-the-fly though. Since you're already using a variant rule, it's easy (and totally true to the spirit of the rules as written) to house rule that such effects last “until the end the next round”, giving a full round plus a few initiative counts of effect, fully covering the opponent's next action.
The only drawback of that is that it might cover more than one of your opponent's next turns, if the initiative counts fall that way. Why do I say that's a drawback? Because the enemy can do the same to you, if they use such an effect against you and the dice go their way. However, that makes it pretty fair: the PCs might occasionally get the double-dip benefit, but so can the enemy. And the dice are likely to more often go in the heroes' favour, since that's the overall design of the rules.
The closest you can get is with the warlock invocation master of myriad forms, that lets you cast alter self at will. It still requires concentration and you have to refresh it every hour. It also needs you to have at least 15 warlock levels.
If levels are at a premium, but only the illusion of a different shape is enough for most of the time, you can go with mask of many faces, that requires only 2 warlock levels and lets you cast disguise self at will. If you take another 3 levels in sorcerer, you will be able to sustain alter self for 4 hours with the extended spell metamagic.
If you are the DM making an NPC, I recommend you just grant them the spell at will or base them on a shapeshifter fiend. It is unlikely to unbalance anything and might add nice flavor to the character. Also, neither at will low level spells or shapeshifting is rare in fiends, so you could consider it an "established mechanic". As far as that can go for a custom NPC anyway.