Wi-Fi is less of a drain than 3G or 4G (with 4G being the worst offender) but leaving Wi-Fi on will still noticeably drain a device's battery. I can almost assure you that charging your phone via USB is not going to adversely affect it. Just because the charge cycle takes longer via your laptop, that's not going to harm your phone. If anything would deteriorate the battery, months of leaving on Wi-Fi, 3G, 4G, etc. during non-use would eventually cut into the battery's lifespan. Even still, if such was harmful I seriously doubt Google and Samsung would've engineered the Galaxy Nexus to even be charged via the micro USB port. I say try turning off your Wi-Fi while you're not using it for a day or two and see how that goes.