IMO, you should only spend more money on higher end parts if you are willing to do it strictly for the added pleasure you'll get from having nicer parts (and don't forget to subtract the stress that having more bling on your bike adds if you lock it up anywhere vulnerable).
I've got the money to indulge myself when I want to. I've gone as high as Ultegra on my commuting/weekend ride bike. Now I've got a dedicated commuter and another road bike for long weekend rides. I've got Ultegra on the road bike and Tiagra on the commuter, with a full Ultegra gruppo on the shelf that I could put on the commuter if I wanted to. So that's my level even with indulgence in mind (and FWIW I feel pretty secure about my lock-up situation at work).
If value was my primary concern (even given limits on my tolerance of sub-par performance), I'd use my beater bike which has an 8-speed drive train with Shimano bar-end shifters. It performs wonderfully and I'm certain it's the most durable drive train I own.