I thought I’d give a few of my own thoughts on the matter.
Before around September 2014 I never would have considered waiting for anyone. After a rather interesting (but probably completely predictable) series of events, I will admit that’s it’s possible for a reasonable appeal for waiting to exist. I still think you shouldn’t wait for someone you’re not crazy about. On the other hand, if someone makes you crazy enough to wait for them, maybe they’re worth it. There’s some grey in between of course, but that’s the general idea.
So. What exactly is it that would make you want to wait for someone?
If they weren’t going on a mission, you’d definitely marry them
I at least hope this is obvious. It seems quite peculiar to wait for someone you’re not absolutely sure you’d marry within a few months otherwise. Surety should be proportional to commitment. In fact, especially for girls going on missions, this case should mean it’s a hard to choose between a mission or (delayed) marriage. If one decides to serve and the other decides to wait, it should be because the mission is the only thing that could keep them apart.
You’ve gotten to the point where it would be extremely awkward if you married other people
This is a rather interesting point. While surely there is some amount of awkwardness surrounding every previous relationship that didn’t work out, there comes a time where you share so many inside jokes, stories, plans for the future, documents in the cloud, friends, and so on that it would be extremely difficult to replace each other. Old memories would keep popping up all over the place. You know this case applies to you when you use certain vocabulary and phrasing and find yourself repeatedly having to explain what it means because only your significant other is familiar with it. Imagine having to explain all that to your spouse.
You feel like you’re cheating on them if you flirt with or date other people
Here’s an obvious sign. This is more likely to occur initially after separation and therefore less pertinent, but if it’s been months and you still feel loyal enough to your missionary to feel guilty dating other people, then that might be saying something.
The obvious caveat here is to make sure your significant other (the one on the mission) feels the same kind of loyalty toward you. If he or she stops affirming love and commitment or just stops communicating altogether, it might be time to consider other options.
There aren’t any viable alternatives
This reason should be used as a last resort, not an excuse to stop looking. It’s good to keep a healthy interest alive while your significant other is away simply because the mission might be part of a greater plan to help both of you find your real eternal companions. On the other hand, if you’re looking and no one seems promising, it’s awfully comforting to know you can always marry your missionary after they come back.
I wouldn’t have ever considered waiting a few years ago, but then again I never encountered these cases years ago either. That’s life for you.
If you’re wondering what you should do, I can’t (and shouldn’t) tell you that you should definitely wait or not wait. I can say however that there are certain cases where waiting is perfectly alright. If you’re both committed and insanely in love, not even a mission can keep you apart.