You and ZV obviously don't care - but the OP wanted to know if there's anything wrong with leaving it off.. It does make a difference. You might not notice it in 6-7 months but give it a year or so and it'll be noticeable.
Yeah, the fact that there's no buildup in any of the cars I've removed it from is clearly just me not caring.
Unless you're going off-roading or ploughing through sandy beaches there just isn't going to be enough junk getting into the engine bay to be any sort of problem at all. Period. End of story. The 951 has been missing its plastic tray for at least a decade now and it still looks clean underneath. The Volvo for maybe only half that, but, just like with the 951, still looks perfectly clean underneath.
Most of the time they are there for noise control or, as Marlin said, to catch fluids so owners don't notice insignificant leaks and panic. If you're running track days at 100+ mph then any airflow changes they make might be noticeable, but on the street it just plain doesn't matter. Hell, I don't know a single 944 owner who still has the undertray on their car and I've never heard of any problems at all because of it.
The simple reality is that for street cars an undertray just plain doesn't make any meaningful difference. If someone wants to replace it because the $20-$100 is worth the piece of mind, that's fine, but driving without an undertray won't hurt a goddamn thing.
ZV