Even though it's only about a mile from the station to the terminal, taxis are extremely expensive ($20-$30).

The shuttle service is not very reliable. It only runs once an hour and although it's sorta timed with the LIRR, the driver is often late, resulting in the worst possible timing.

This might not be a problem for you (some people try to arrive at the airport hours ahead of time), but I usually fly short trips to BWI to save on time over driving. I don't even check bags because I think it takes too long for baggage claim; So having to wait an extra hour for the next LIRR train because the guy driving the shuttle bus was twiddling his thumbs between rounds for 65 minutes instead of 55 minutes is no good for me.

For a 5-day trip I say go for it. If you're worried then buy a ticket for the LIRR when you park and put the receipt in your window next to your inspection sticker. Save the ticket for later or return it after you fly home.