My name is [name]Rebecca[/name] and I absolutely [name]LOVE[/name] it. I HATE when people call me [name]Becca[/name]. It reminds me of a masculine name.
Bex is a great nickname for [name]Rebecca[/name]. If she prefers [name]Becca[/name], that’s her choice. But I think [name]Rebecca[/name] is a classy name, and has a lot of potential nicknames 
To all the people who know someone who only goes by [name]Becca[/name], what’s the point in [name]Rebecca[/name]? Obviously they prefer [name]Becca[/name] and not their full name
[name]Lucy[/name], which many berries love but I can’t stand, is a nn. Why is that OK? And why is [name]Leo[/name], [name]Jack[/name], and [name]Kate[/name]? I rarely see anyone say “go with [name]Lucia[/name], [name]Leonard[/name], [name]John[/name], and [name]Katherine[/name] instead”.
There are a lot of nn names I hate but there are many that I like. I wouldn’t go with a full name that I don’t like just because it’s “classic”.
I say “[name]Jack[/name] and [name]Kate[/name] are nicknames” all the time, though. I like [name]Jack[/name] very much as a nickname but I’d never use it as a full name. [name]Do[/name] I get points for being “annoying, but consistent”?
[name]Lucy[/name] is more of a translation than a nickname. It’s older than many of the names (like [name]Lucinda[/name]) it is used as a short form for, today.
I recommend putting the formal name on the birth certificate. Almost everyone will assume that a [name]Becca[/name]'s real name is [name]Rebecca[/name]. If not, she will end up explaining to numerous people that her name is “just [name]Becca[/name].” People are likely to express surprise about it, which may make her feel badly about her name. For job applications and career advancement, she is likely to be taken more seriously if she is “[name]Rebecca[/name]” rather than “[name]Becca[/name].” Bottom line: It would not be the end of the world to use [name]Becca[/name], but using the formal name would have benefits and eliminate potential regrets and confusion.
Taz says it perfectly
I agree with most of the pp’s - [name]Rebecca[/name] is a classic, pretty name that has the wonderful built-in nn of [name]Becca[/name].
I’m a [name]Rebecca[/name] and I appreciate that I have the freedom to go by my full name, which I do. [name]Becca[/name] to my close friends.
My parents weren’t psychic and wouldn’t know what I prefer.
Another vote for [name]Rebecca[/name]. It’s my sister’s name and she uses the full name, [name]Becca[/name], and just [name]Beck[/name]. I think she was [name]Beckett[/name] to some of her friends in highschool.
While I would be willing to use a proper name I just sort of like to get to a nn I love, I wouldn’t be okay with picking a proper name I flat out dislike with the same goal. I love [name]Rebecca[/name] (especially [name]Rebekah[/name]), so it’s the one I would pick without hesitation, but I don’t think [name]Becca[/name] is unusable as a standalone. It’s no worse than [name]Bella[/name], imo, and better than many other, more babyish, nickname-names. I think it would be okay if your friend used [name]Becca[/name] on its own if she really finds [name]Rebecca[/name] that unappealing.
Thank you so much for all your opinions, I’m happy I’m a part of the majority who prefers [name]Rebecca[/name] over [name]Becca[/name]. I’ll give a link to my friend and I just hope she’ll change her mind.