I went through a similar discussion with my spousal unit. Ultimately she had valid points, I start things but don't follow through; what was I going to do with winter coming; why spend money we don't have and then let the bicycle sit if you drive every day; you already have a bike that works...but I really wanted the new bike.

Approaching the decision like a business issue, we agreed the best bet is to run a proof of concept pilot program. Either use your current bike or by a used one and ride the 28 miles for a few months, rain or shine, heat or cold, regardless of the time of day for an an agreed upon period of time (6 months in my case July 1 through Jan 1). Believe me you'll decide whether you really are committed to the whole commuting routine or not.

If after an agreed upon period of time you are still commuting the agreed upon amount (in my case 4 days a week) buy the bike you want. In addition to validating whether you are going to stick to it, you'll have a better idea of the bike you really want. In my case I bought two bikes, a Trek 700 that I stripped and overhauled and a Trek X0 frame that I built up as a compact road bike with Campagnolo wheels, shifters, drive, etc., with extra cyclocross wheels I have the best of both worlds, a commuter that is classic steel frame and a compact road bike that can cyclocross, and a wife that at least tolerated the expenditures.

But, back to the original point. It doesn't hurt to prove that you're serious about this whole cycling thing before dropping a lump of money, particularly if money is tight. You don't want her to see your bike sitting in the garage every day while you're at work and fuming about the money you spent and aren't using. Trust me, that can be unpleasant...