I feel like entering a sitcom script into a competition with the sole expectation of getting a commission out of it at the end is not the best way to approach a comedy writing career. If your end goal is getting your series made, entering competitions will help but it's not the only thing you should be doing.
To me, things competitions are useful for are:
-getting yourself into the habit of meeting deadlines and actually seeing projects through to the end
-generating a sample script which you can then send to people (whether you make the longlist or not) be it production companies or agents -who you should be contacting/sending stuff to anyway, competition or no competition
-if you do make the longlist/shortlist, even if you don't get commissioned on the spot (imagine!) it just looks really good on your CV/personal statement (which you are also sending to lots of agents, not just ones who may be linked to the competition)
-I think I'm correct in assuming it is generally just good for networking/has lead to other writing jobs for previous finalists (even if these jobs aren't necessarily their own 'pet projects', finalists have gone into writer's rooms/I think a couple of SM finalists have gone on to write for cartoon network shows?)
I've entered the Sitcom Mission a couple of times; never made it to the final (not that I take it too personally*). I've never regretted entering, because I like writing scripts and having samples ready to bung over to industry-related people when they ask if I have anything I can show them. I also like seeing other writers I know (or whose names I recognize) do well!
I think it also helps that I have had some writing work through other methods and am not pinning all my hopes on this/other competitions. But I do think it's important in general to be doing other things/having a wider view than just competitions, otherwise, statistically speaking, not enough people will see your script (or even be aware that you are out there). It would be like if you wanted to be a musician but just auditioned for BGT, instead of also booking gigs or functions/playing open mics/plugging your SoundCloud/Bandcamp (MySpace??) -you also just have to put the work in, to an extent.