There is another element and that's "do what you enjoy most." For example, you wouldn't be playing Ein at all if, say, playing Hayate "kept working" better. Or you could just be a tier whore and pick whoever's "top tier" and get things working more. It just depends on what style of play suits you.
For me personally, turtling just isn't fun. The appeal of DOA for me largely lies in the fast, flashy combat potential, and turtling/keep out tend to be the same few moves used much more sparingly (usually not the cool-looking ones). The last thing I want is to hide back and stand around, waiting for someone else to make the first move. So for me, it's just boring. If that's your thing, go for it. But I will chase your ass down because I'll get a literal knockout of boredom hanging back all day.
On a competitive level, I also just like to stay in people's heads. Letting someone turtle gives them the chance to think and I don't want that. I want them sucked into a blitzrieg of never-ending hell from match-start. Even with characters who aren't expected to be rushdown characters. If anything, that unexpected angle just makes you more intimidating. So personally, it's not my thing. But to each their own.
All my life I've been a player that always loved rushdown, a style that is lightning fast, constantly in your opponent's face and giving you a rush throughout the fight. I still enjoy that style but the fact of the matter is not all characters or even games cater to that style. As far as DOA goes, I ended up gravitating more towards characters who space and play fairly defensively.
Turtling and spacing are two different things, but people seem to confuse that. Turtling is constantly backing away from your opponent until they eventually approach you, then you attack, probably opening them up and frustrating them, then continuing to do the same thing. That's not what Ein does (or I do for that matter). Spacing is about keeping an opponent at a certain distance, watching their movements and planning/reacting accordingly. I dashed in so he did too, then I back dashed real quick out of his range and 236Ped him when he whiffed his move, but if I had just rushed in he would have punished me for it, that kind of thing.
Now, strictly speaking for Ein, I always played him upclose, I spaced with him against people who did the same to me, but I loved being in people's faces, and always was in 4. But the simple fact is TN took away the tools he needed in order to really compete with the greats up close and he simply can't handle close quarters with certain characters, he has to space and in some cases yes, turtle in order to come out on top. For me it's thrilling fighting against an opponent that knows and is aware of the spacing game, it makes the game nerve wracking in a different way, two players on edge, fishing for that opening. (My DOA5 Vanilla "Akira vs Ayane" videos in the media section are the perfect example).
Point is, you do what's necessary, weather that's relative to being fun is up to you. Spacing with Ein is fun, spacing with Akira is not, he's a character that needs to be up close in order to do damage, so rushing down with him is a blast. It's all character dependent. The idea of rushing down with a character that isn't made for that could be thrilling, but the idea is also generally flawed, and against a player who knows that character and knows that weakness, you're going to have a hard time and will either be forced to switch your tactics, or switch to a character that accommodates your playstyle.
...For the record though, no turtling isn't fun. It's annoying and I don't do it unless there's like 10 seconds on the clock and I have a life lead. But a lot of people confuse it with spacing and legit defense.
I'm not gonna lie though when there is like 5 seconds left on the clock and I have the life lead I'm backing up lol. Come try to deplete my life with 5 seconds left when I'm Ein of all people. 
