I'd agree that Parallels cannot replace the bootcamp option. It simply can't in terms of compatibility and performance.
However, that doesn't mean Parallels can't be a great asset for a gamer on a Mac. As also mentioned above, it is great for classics such as most GOG games. I found it more convenient to setup and mod Planescape Torment, Icewind Dale and Icewind Dale 2 there for example. I have them running with widescreen mods, etc. in a Windows XP virtual machine with Parallels 8 and they work like a charm. I also tried Painkiller and Serious Sam II, which despite being from 2005 I think it was, both support widescreen resolutions and they run perfectly with Parallels in that same WinXP VM.
Not everything will work though. I tried a Prince of Persia game, a relatively recent one but I forget which (sorry) and that ran at a glacial pace even though it is nowhere near being a new game. So it does vary some but overall for older games I find it more convenient at times that using Wine or for GOG MS-DOS titles, more convenient than moving them into Boxer since they just install and work in a Parallels VM.
I have not tried more recent stuff but it is my understanding that some games work just fine although you will need to back the settings off some compared to running with bootcamp. This is going to vary a lot of course according to your own system specs and how much resources you allocate to the virtual machine.
By the way, again going back to classics, you probably know that some oldies don't like multiple core CPUs. It is very convenient to switch the VM to single core as desired for such titles to run perfectly.
I would not dismiss Parallels as worthless out of hand at all as some do. It most certainly has its uses and offers added convenience for a Mac gamer, particularly one who also plays classics.
For me who is currently using version 8, they are offering an upgrade to version 9 now that includes a free upgrade to version 10 when that comes out. I'm sure this will be timed with the release of Yosemite. I am going to buy into this. I find it very useful personally even though it does not eliminate all reboots. It most certainly does offer me added convenience and does eliminate some reboots.
One last thing of note. Parallels does not currently support DirectX 11. So any games requiring that are out. I see it as a non-issue though personally. Anything that new is better run in bootcamp I think. In version 8 that I have, DirectX 10 support was experimental. From what I understand it was improved in version 9 but that's all I can tell you about that. Again, from a performance standpoint I'm not sure if it is a moot point or not. DirectX 9 support is generally really good though.
Hope that helps. For me it is one more tool in the kit along with Boxer, Wine and as a last resort bootcamp although personally I always go for native games and have a lot of them now so I'm focused on those. It's awesome not needing to care about compatibility, etc.