Well, one sole ability of wizards that is lacking in 5E is the option to research spells, so they don't have to find them. And, since there is no limit to the number of spells a wizard can have in his book and thus learn, there is no such thing as a useless spell. Will it be worth the money to scribe it? That is up to the player, of course. Since it is also downtime, I would not be concerned with the time required to scribe a spell, and IME most games have enough gp that I would not be overly concered with the cost, either. Of course, your experience may differ.
If spell versatility makes it into an official book, it is always still optional, just like everything in 5E. And finally as I said, once the downtime is over and the adventure is underway, I can't see much chances for extended long rests to permit multiple spell swapping. Even so, I think most of the time we are talking about a handful of spells at most, since most players choose spells that they think (and often find) are most useful from the start. In our games, characters rarely even swap out spells at level advances other than to trade-up to a higher level spell.