It depends on the battery. Certain technologies have memory, others don't. So it is best to check what yours is and see what is best for it.
NiCd battery's have that so called 'memory effect' meaning,.. if you dont decharge them, before you recharge them,.. you'll lose quite some of their cappasity over time.
the only 2 battery types i can immagine beeing used are NiMh and Li-ion batterys.. Li-ion will be the most common one.
the only way of wearing them down really fast if you literairy overcharge them. but like Omega said,.. a Laptop has a charging circuit build in,.. and as far as i know, it switches over to drop-charging ( i dont know the exact englis word for that type of charging,.. but in dutch its called druppel-laden,.. and drop = druppel..

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meaning, it only charges that much to keep the battery fully charged without overcharging it.
knowing that, i really dont think it'll make a difference.
ofcourse,.. you save the higher energy bill if you unplug the adapter.. because even when its not charging... the adapter itself consumes power.
(this is what i know by myself... i didnt look anything up... so im not 100% certain of every single word,.. but ill find out soon enough if something isnt 100% correct ofcourse

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