This is really about the only reason that you might consider it. At the end of the day, it really doesn't matter if you've taken it or not, unless it's a requirement for employment.

There are plenty of folks out there that get on at the airlines without ever cracking the ATP study guide. Why? Because you learn a lot of that stuff when you're actually at your first airline gig. Granted, you should have the instrument procedures and regulations for flying down, but most interviewers don't expect you to know 121 stuff unless you've been in that environment. You can study for the interview just as effectively with Gleim books for the Instrument, CFI, and Commercial writtens.

My personal opinion is that most folks looking for their first job at a regional probably don't have a spare $80 to toss away on something that they really don't need. If you do, go for it. It won't hurt, but it isn't necessarily going to help either. (FWIW, I didn't get my ATP written done until two weeks before my ATP ride, which was after 4+ years of flying at an airline.)

If you don't get it, it probably won't hurt you (especially now with some airlines hiring at 250TT/25ME). I wouldn't worry about the FEX at all. It's just a waste of money, and by the time that you ever looked at needing it, it would be expired and you'd have to retake it (Or at the very least, you'll want to retake it... I don't know of any regionals that have Flight Engineers, and the instant that you leave your regional for a carrier that has FE's your results are probably going to expire).