Does keeping a mobile phone plugged in increase its battery's lifetime?
Li-ion batteries degrade faster when fully charged. If the charger keeps the battery fully charged then it could reduce lifespan by half or worse, particularly at high temperature. A battery that loses 5% capacity per year at 25&deg;C when half charged might lose 20% pear year when fully charged. The higher voltage at full charge may also make it more susceptible to 'puffing'.   

Charging and discharging the battery wears it out, but it can last longer if not fully discharged. A battery that only gets 300 cycles from 100% to 0% might get 3000 cycles from 75% to 25%.     

If you are using the phone a lot, can supply external power while using it, and can prevent it from fully charging, then battery life could be extended. But just leaving it plugged in on standby won't help much because current draw is low so it isn't wearing out very fast anyway, and if you can't prevent it from fully charging then lifespan may actually be reduced rather than extended. 

How to Prolong Lithium-based Batteries