A lithium-ion battery is spec'd to have a certain number of charge/discharge cycles, based upon the charging current. Higher currents have the effect of reducing battery life. Having a weaker charger limits the current, and how fast the battery can charge, therefore extending the life of the battery at the expense of longer charging times.
For example, some of the Motorola phones come with 800mA chargers, but will consume up to 1.6A with an aftermarket charger. On the other hand, a Blackberry tour may only draw 1.1A. As
JONNIE_R mentioned, having more power available won't result in greater currents utilized by the phone.
-Charles
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