I've used smcfancontrol for years, excellent app. Don't really need it on my mba as it stays cool for my usage anyway - love how, with coolbook, my mba runs 1080p video at 59 degrees C at default 2000rpm.
However, it's essential imo if you use handbrake or play games etc. I echo what b-rad g said, the fans on my late 08 mbp wouldn't fire up to max when it really ought to, whereas smc easily enables you to prevent your cpu hitting 85 degrees plus.
Plus, personally I find it reassuring seeing the temp and fan speed permanently in the toolbar.