It costs more to get the same proformance and having it wear out is more of an issue as you cannot change parts like you can in a desktop (if the video card dies, it is very likely that you will need a new motherboard).
Yeah, I hard to learn that the hard way actually. That very problem mostly happened to me*.
But if you pay the money (or can accept that it will not be able to handle new games) and take care of it (external cooling pad is recommended while gaming) then it can work out. Just do not leave games running while you are not playing.
For the most part I'm not interested in a lot of "new(er)" games; there's very few current or up-and-coming titles that I'm very interested in, whether due to lack of appeal or publisher/developer reputation.
One thing I will note is that I do benefit from having a cooling pad, so that's good. Probably the biggest thing for me, though, is manufacturer. The laptop that I used to use for gaming (I've got 2 total) is an HP, and ended up borderline-frying the video card. I still use it, just not for gaming. The other laptop I have, a 4/5-year old Toshiba, I use for gaming ATM, though it's mostly my older or lower-hardware requirement games (like Sins of a Solar Empire 
). Considering I had the HP for all-of a single year before it quit on me for gaming, and that the Toshiba still works pretty well in everything I use it for, has indicated that a Toshiba is going to be a major laptop manufacturer of choice for me.
*I say "mostly" because the HP still works pretty well, if running a bit hot for my preference. It also happens to be one of those laptops that has a motherboard-integrated video card. Not cool when I found out.
A high-end Desktop graphic card is physidally almost the same 'volume' as an entire Laptop. They're not that big because the designer is lazy/generous...they're that big to cope with the power consumption and subsequent heat generation.
Shrinking one down to fit into the limitations of a Laptop form will typically require [obviously] power use reduction and throttling back of clock speeds to reduce heat as well.
To overcome those limitations a mobile card would need superior chipset architecture [read smaller] and attract an even more premium price.
In other words, bang-for-your-buck in gaming terms means Lappys are never the option of choice.
See if you can find comparison data/specs on a card/chipset that has both laptop and desktop versions...and compare their performance [and price]. That should make it clear...
That first part would explain the enormity of the ATI card Island Dog showed off in a post of his, when he installed it on his desktop. Probably the big thing for me, though, is that I want a laptop that can game very well, mostly because I like being able to up-and-go with my computer. A lot of this has to do with the fact that I do a lot with my computers; game, write, read, social-network, store pictures and video, listen to/store music. So I generally want something that will hold a significant amount of information that is important to me, and also be something I can pack up in a good five or ten minutes, without resorting to going and getting the packaging.
I'm not bashing desktops either; I actually looked at a couple of Acer towers, but none of the ones that were in the budget that I've appointed for the purchase had what I needed; a discrete graphics card of respectable capability. They also had the slight issue in not coming with a monitor, and the monitors I looked at drove the price up nearly 20%. Then I look at some other Toshiba laptops besides the godlike Qosmio series, and find a good, 800 dollar Satellite-series model that has the card specified in the title. Even if it doesn't come with things like an separate mouse or headphones, I've already got such items.
Another reason I have a bias towards laptops is that, at present, space around my desk is at a bit of a premium. As in, "I've got no idea where I can stick a tower that won't be either in the way or liable to be clogged with dog hair and/or dust. A laptop has the benefit of sitting on my desk, in a fairly compact volume, with the price that I pay being in some performance capability. For what I'm looking for, I'm perfectly willing to take the hit, for reasons I mentioned above.
