I posted earlier reagarding the pros and cons of Southwest or Frontier airlines. I received great feedback. The absolute best schedule would be to take Frontier to Louisville and Southwest home. It's the same price as taking Frontier roundtrip and a little cheaper than taking SW roundtrip.
Is there any reason why this a bad idea?

no

No reason I can think of.

Southwest's airfares are based on one way rates. So, if one way is $99 round trip is $198. Therefore, no penalty to booking the leg home. Is Frontier the same? Or are you comparing Frontier Roundtrip with Southwest Roundtrip rates to make your comarison? If so, be sure Frontier's oneway fare works in your computation.
Debi

We just flew JetBlue (a favorite) from Ft Lauderdale to Laguardia and AirTran from Philadelphia to Ft Lauderdale.

JetBlue doesn't fly from Philly and we took what worked. No issues and all planes were on time.

None. Many do this all the time for convenience and sometimes big savings. Some search engines are better than others at setting this up.

Frontier's one way fare is less than SW for this flight. Frontier r/t would be the same as Frontier one way, SW one way, but we'd come in at 11 pm instead of 8pm. Kid had to go to school the next day so the earlier arrival on SW works better. If Frontier came in earlier I'd just book them for the whole trip. I guess this will be good way to compare these 2 airlines.

When I flew Southwest to Chicago and took the train back, I worried my one-way ticket will be a red flag, got ready for a strip-search! Darn it, they were not impressed and nobody asked anything

In mid-October I'm flying to my vacation destination on Alaskan, coming back on Southwest. Alaskan schedule is much more convenient flying out of another aiport, and no one-way penalty.

I travel on two different airlines for many trips. I do it for the same reason you are doing it glimmer2.

No problem.

The only thing to consider is the cost. If the two flights look reasonable to you, flying two airlines to take advantage of better schedules is a good idea.

One thing to mention though, is that while there are no issues with flying two separate trips (going/coming), it is not always good to fly two connecting legs of the same trip on separate tickets. This is because if your first flight is delayed and you miss your second flight, the second airline will have no responsibility to rebook you.

You can get around this by booking both airlines on the same ticket...eg either have one of the airlines book the whole thing, or go through an agent like Expedia.

I don't think you need to book this throuhg anagent like Expedia and pay their additional fee and you cannot book a Southwest flight through Expedia anyway.

If you are only flying one segment each way then whether or not the first one is late is irrelevant to this particular situation.

The only negative is that you will probably be singled out for individual screening at the security check points.

Keith

The chance of getting SSSS'ed for screening may be higher with an one-way ticket; but the overall rate is still very low. Otherwise, the TSA will be very busy at all Southwest Airlines airports.

The example of flying two legs was to point out how a different situation might not work the same a glimmer2's.

We did this on a trip to Las Vegas this past March and our son was singled out for special screening. It was awful. He was in the gate area waiting for us to finish regular screening for over an hour. Wish I had to go through special screening so I wouldn't have had to spend all that time in a crowded line.