If you're shooting "skeet", instead of shooting skeet, then I'd take it out...meaning if you're just going out behind the woodshed and your buddy and you take turns kicking clays for eachother, then take it out. BUT, if you're playing an official game, it's irrelevant, because you can't even load three, let alone more than that...I personally have changed my springs and replaced my plugs to make sure I COULDN'T load more than two in my skeet guns...then I upgraded to a SxS (don't like O/U's much) and it really became a moot point.

To answer your question in general, No, it won't effect your gun, unless your 120 is pretty old and has been used a lot, or left loaded for extended periods of time...the only mal-effect then is that the spring might be aged into it's state of compression, ie. when you take out the plug, you don't get positive feeding on the last few rounds, since the spring is "worn out". It'll feed fine for the first few rounds, since it would be as if the plug was replaced by shells, but after that it might not feed well...

But that is an extreme case, and not very likely with a gun as young as the 120 (or 1200).

It's easily fixed by calling Numerich or Brownells (or your local gunsmith) and buying a replacement spring for the 120, 1200, 1400, or 1300, as I'm pretty certain they all use the same mag tube/spring..probably cost you more to get it shipped than the spring costs.