Sexual attraction is only partly about a pretty face and a nice body. A person also has to get our minds racing in some fashion. For T-Rex, one extra "oomph" is appealing to his love of flappers and old timey fashions. (I'm sure there are others. What T-Rex likes in a lady has a fair amount of canonical continuity.)
Maybe you know all of this already, that the brain is the largest and most powerful erogenous zone in the human body. You should keep reading anyway.
I enjoy looking at naked ladies as much as the next heterosexual dude, but I am equally interested in what she is wearing before she gets naked. I admit that it takes some amount of skill to artfully photograph a naked lady, but it takes more skill for her to start out dressed fashionably, then have her remove the clothing in a sexy manner with a satisfying progression. The lingerie industry exists because of this.
I work for a print shop that is basically the McDonald's of print shops. We print a lot of stuff for a lot of designers, and design students, who for some reason are often incredibly attractive. Quite a few of them brighten my day just by being in the store. There is one, though, who isn't that attractive physically, but I'd rather go on a date with her than any of the others, based solely on her artwork. One piece had a giraffe looking out a window, with a bunch of flowers growing outside the window. Another had a unicorn, a rainbow, and an accordion all in the same picture, which is a combination that makes my brain say, "Fuck. Yes." There's another girl (not a designer, as far as I can tell) who, in addition to being stunningly attractive, has a bitchin' haircut and a lot of beautiful tattoos.
Now, I don't want you to get the wrong idea here, gentle reader, that I am the kind of person who judges a woman's worth based solely on whether I find her attractive. It's just that checking out ladies at work is the only thing that breaks the tedium, and otherwise I am just standing there staring off into space. If you have worked retail for for than a few months, you probably know what I am talking about.
Getting back to the comic, T-Rex's rating policy sounds a little silly, but he definitely has the right idea: if 10 is going to be your maximum, you should only give a 10 to those women who are stunningly gorgeous, and excite your brain. Otherwise, you have to start raising your ceiling, and say, "Oh, she's an 11 out of 10." Which is ridiculous. It's not even funny. "Why don't you just make 10 a little hotter?" "But... she's an 11."
It's interesting that T-Rex labels the extra point five as being for "emergencies." As if there could be a sexiness emergency. Okay, yes, there are definitely sexiness emergencies... and now that I think about it, this is not that bad of an idea. I should probably develop a sexiness emergency plan, because so far, my reaction mostly involves becoming speechless, then either trying to pretend that there is not a sexiness emergency, OR to try to think of something witty, clever, and/or sexy to say. So, perhaps I will spend this weekend coming up with a "Sexiness Emergency Plan."