Well, as long as you actually want to be a full-time scriptwriter, obviously. I wouldn't bother if you want to be a train driver or milliner or something, it's at best going to be taking the long way round.
I'm biased in favour of script comps (as they're known within the industry, probably), because I won a Channel 4 script comp in 1999, which immediately led to work on Bob The Builder and Smack The Pony, a castle in Scotland and membership of the Diogenes Club in London (look it up), but even if you're never going to be quite as successful and gorgeous as me because of your inferior genetic code apart from anything else, it's still worth entering any script competitions you come across (with the caveat at the end of this post about not paying through the nose for the privilege).
One of the things that puts people off is they're seen as a bit of a lottery. If it's well-publicised, a script competition could get as many as five thousand entries, which makes you think 'cuh' and 'whaaaaaaat', and 'well what's the point, frankly'. Now I'm pretty sure this is going to be the top end: Twitter types have told me more recent competition have brought in more like just under two thousand, but let's go with five thousand just to show even with this amount of entries, the volume of real competition isn't necessarily as imposing as you might think.
So let's look at an imaginary script competition. The broadcaster is running this in connection with the National Forestry Service, and they've asked for an hour-long television script for a new detective series, ideally set somewhere wooded, with the only restriction being that none of the characters use an axe at any point, because that makes people who like trees feel sad. Note: THIS PROBABLY WOULDN'T HAPPEN.
OPENING NUMBER OF ENTRIES: 5000
Right, let's do the first bit of maths. Of those five thousand, the first two thousands of those entries will be written in a format utterly unsuited to television scripts, quite possibly scrawled on cardboard, in green crayon. One writer, in an attempt to ingratiate themselves with the competition organisers, has handcarved their entire story onto recycled floorboards, delivering this to the broadcaster's address via wooden glider. Fortunately they have laid the script out according to the industry standard! Apart from using comic sans, d'oh! The remaining thousand entries will be poems about cats.
REMAINING SCRIPTS: 2000
Now we've got a stack of things that actually look and feel like scripts. Unfortunately (or fortunately from your point of view), at least half these can once again be rejected almost immediately on the following or similar grounds:
1. Instead of the asked for hour long detective drama, the script is instead a half hour sitcom about working in an airport newsagents.
2. The script is a ninety minute feature about talking furniture, turned down three times by Pixar, although there is a tree in a flashback sequence where the footstool remembers his simple country upbringing, which is why the writer thought it might be worth a punt.
3. The script is an forty minute radio play, with the phrase 'here in the woods' cut and pasted onto the end of each line of dialogue (in a different font).
4. The script is clearly about a vet, but the word 'vet' has been search and replaced with 'detective' apart from one case, where 'vet' was misspelled 'bet' and thus missed.
REMAINING SCRIPTS: 1000
We now have over a thousand hour long detective dramas with at least one scene set in woodlands. Sadly, at least half the writers didn't read the brief properly, and have the central murder committed with an axe.
REMAINING SCRIPTS: 500
Now we have a few hundred scripts that are each an hour long, are clearly in the crime genre, have at least one scene set in the woods, and have no murders committed with an axe.
Sadly, at least two hundred and fifty of these are thinly-veiled attempts to rewrite the one script the writer has been working on since two thousand and three, and has been set, variously, in: a space station, an abandoned tube station, the Empire State building and a shed in Margate, depending on which script competition they're sending it in to.
Every script reader involved in this competition (and they do read the scripts, they're not picked out of a tombala or anything, apart from some comedy ones I didn't win, where the actual winners were clearly picked out of tombola) has come across these scripts before, and has had puzzled conversations with their colleagues about why the writer, who is usually not completely without talent, hasn't, say, tried to write something completely new. But they haven't, and consequently, those scripts end up face down in a special pile, each with a sad face emoticon drawn on them in biro.
REMAINING SCRIPTS: 250
Now we're talking. Two hundred and fifty scripts remain, each one a finely-crafted expression of the writer's vision, with a plot that ticks along like a well-oiled machine and characters that feel vibrant and alive. It's probably not two hundred and fifty, to be honest, it's probably about a hundred. There's nothing more I can do for you at this point, you're on your own.
I was going to write a bit about not paying for the privilege of entering a script competition, until I remembered the C4 competition I entered did, I think, have a £10 entry fee, so that would have made me a terribly hypocrite, which I am sometimes, but, you know, I don't want to by a hypocrite about it. But I really wouldn't suggest paying more than a tenner to enter a competition, and even then only if it's a broadcaster or production company you've actually heard of.