I think it is acceptable to wait for a search warrant to search someone's house, whatever that person was doing there, chances are that there will be traces of it even if the person tried to destroy evidence (blood residues, fingerprints, smells, etc).

But for a smartphone, I think it works a little bit differently. Any evidence that you might have on your smartphone can be quickly destroyed without leaving any traces. Even if a search warrant takes 5 minutes to be issued, by the time it arrives to the police it will be completely pointless, since you were able to press delete on all your photos during those 5 minutes.

So, what, if you find out you have a warrant, you'll immediately destroy any evidence you have ? Why not, y'know, destroy it in the first place, before any warrant is issued ?

I don't understand the point you're trying to make.

If you're worried about someone destroying evidence, you need to detain them and confiscate their phone. Until then, you have no idea where the evidence is, and it could be destroyed at anytime no matter where it is.