Having a yandere has(believe it or not) benefits, but of course depending on the level

Lower than 3 should be a good level of protection, but higher, then i guess you have an incredible bodyguard.

If your yandere is male then he would of course sense danger and stay with you at all times, however the females would wait for a attack, and protect you.

The chances of a happier relationship
Heres an example
Yukki: Im certian Yuno will love these flowers
Yuno: Yukki!
Yukki: Yuno!
Yuno: Yukki i-... Who are those for?
Yukki: Their for you
Yuno: aaww thank you Yukki -kisses Yukki-

Now before Yukki got flowers for Yuno, it's obvious what she did. A good relationship should be where the yandere is doing his/her job of protecting you, while you are making plans of showing affection towards him/her.