I don't know about this really. But just googled and read there can be some confusion about it -- maybe it's more helpful for the battery to take it out IF you run anything that causes the machine to really heat up. Though that probably actually includes me in the end, once the 3-d games come out. 
When it comes to battery maintenance - it depends on the type of battery and the model. Some laptops when on power run in a reduced CPU mode because it does not have the additional reserves to draw upon as needed if there is a spike in demand.
While I cannot say this with any certainty that its 100% accurate, most devices are designed around a charge / recharge cycle. Often a constant charge with no discharge will reduce the battery life over time.
My home laptop has a cycle count of 463. Which means that over 4 years this laptop batttery has been completely discharged 463 times. (2x 50% battery level = 1 cycle) This is 2 years of heavy charge/use at work before it was replaced and became my home laptop. Since then its usually used on power for media etc while the newer more powerful device is used at work. Most non-time limited battery warranties are built around a 300 cycle count. A full discharge each work day (5 days, 48 weeks per year) is 240 Cycles.
Laptops are optimised for disconnected use - so warnings kick in when a batteries performance falls below what are often seen as acceptable parameters for disconnected use. But if your view is that it gives you time to save and shut down in a power outage - then these battery warnings are irrelevant unless a power outage doesn't give you this window.
I guess what it depends upon is why you need a device with a battery - is it for redundancy - when the power fails - or is it for disconnected use.
For me, I try to ensure my work device is capable of a full days use (unless I jump into a power use mode which really draws on the battery) without mid-day charging. I have a 12 hour battery - but if I am running 1 or more Virtual Machines, or doing heavy work I often need to plug in a charger before a full workday is out. If my work load is lighter, or focused differently I can sometimes last 2-3 days without charging.
My heavy work use however, would most likely be a lighter load than dedicated 3d gaming unless its the occasions when I have two different VM's running.
But do you need to consider replacing the battery? If you use it plugged in 95% of the time, and only consider the battery as a backup in an outage to let you save - well if it gives you 30 mins + - then no you don't need to replace it. But if you use it on the road or on battery and are getting less than an hour or two (if its a modern battery designed for 6-12 hours use) then yes you do need to replace it.
Another thing to note with laptops. If you take out the battery - can you still use it when plugged in to power?
If you are not concerned about battery use, and don't care about immediate loss of function in a power outage - then you don't need to replace a battery on a device permanently connected to power.
I haven't seen a laptop yet that wont function with a dead battery as long as they are connected to power.
