You can, but then you're not testing IE7, you're testing IE9 in IE7 mode. That's a condition which may be worth testing in some circumstances, but it's not the same as testing IE7 properly.
Yeah I actually had to learn this the hard way. Clients complaining about things not looking right in IE7/8 when I tested using IE9 in IE8/7 mode. You need the actual browser to be sure you've tested everything.
