Mobil devices seem to me to be less secure because (1) who wants to have a password on a smartphone when a call comes in?
First, "mobile devices" encompasses more than just telephones. Second, LastPass has its own login separate from that of the mobile device, itself, so there is no LastPass-imposed password prompt simply to answer a phone call.
(2) one uses networks that are possibly insecure when on the road
Given the measures employed by LastPass, itself, any perceived difference in how "insecure" the networks possibly are, in the different contexts, is negligible. In other words, if public networks were "less" secure then home networks wouldn't be secure enough - but they are.
(3) more likely to have a tablet/smartphone/laptop stolen when on the road.
Ditto: The level of security that needed to be offered by LastPass had to be so high that the difference between having a device with your encrypted password blob on it stolen and not having that happen had to be negligible, or LastPass itself wouldn't be secure enough.
If I did have Lastpass on my mobil device, I would just turn it on just to logon and then turn it off
That's generally what many people do, as far as I know: They boot up LastPass as they're web surfing and using apps, and then shut it down. I have it set up to reprompt for a secondary (short) password each time a password is made available to me via password-specific paste, as well as every X minutes.
as the vault makes all your passwords visible (by clicking on the "eye" on a password in the entry form).
I have it set up such that that requires a re-entry of the full vault password every X minutes... I think 5 minutes.
Logging in on a cell phone connection probably is more secure then a hotel wifi.
Again, if there was actually a significant difference vis a vis LastPass in this regard, then LastPass wouldn't have been secure enough for any use.
I am far more concerned about my passport being stolen when I travel.