[Liversidge]

My local group play the Weak No Trump and use the 2S response as a weak takeout - opener bids 3C and responder passes or converts. Andrew Robson recommends that method to his students on the grounds of simplicity, and qualifies it by saying it should be used with weak hands only, and that it should be at least a 6 card suit with a good honour. His example hands have no more than 6 honour points.
On a number of occasions my 1NT has been taken out with 9-10 points when it would have been better to just pass and play in 1NT. I think the sight of a goodish suit - AJ9863 makes some players to want to show the suit. I am often asked what 'weak' means in this context. My personal guideline is no more than 6-7 HCP, i.e. where we are likely to have less than half the points - if we pass with around 21 HCP then for our opponents to get us down they need to make 7 tricks in no trumps with just 19 points. Can you give me some steer on this. (I appreciate that there is more to it than point count).

[Cyberyeti]

There are two dangers of playing in NT:

Opps cash lots of tricks
You can't cash your suit for lack of entries

So how do we look at this:

If you have a 6 card suit, you will have 2 doubletons or a singleton. That obviously runs the risk of being opposite partner's doubleton. I would usually move with a singleton.

The second one is more difficult:

Visualise 3 8 counts with the same club suit

Ax, xxx, xx, KJ10xxx
xx, xx, Axx, KJ10xxx
Qx, Qx, xxx, KJ10xxx

Most people unless they have a lot of diamonds, lead a major against 1N. Hand 3 has obvious issues with a possible lack of a late entry, say partner has any doubleton except AQ and you will see the issue. Hand 2 may suffer from opps cashing a load of tricks, but may well provide a late entry, hand 1 is borderline, if they knock out A♠ early, this may not play well in NT.

The second consideration is tactical, by bidding 2♠ you make it easier to compete in spades if double for opps shows spades, but more difficult to compete in hearts. If I have 3 spades and less hearts, I might bid 2♠.

The third consideration is that 2♠ used like this is wasteful, you should assign a meaning to 1N-2♠-3♣-3M (for us these are game forcing hands at least 5-4 minors).

[helene_t]

If you are short in hearts, especially at all green vulnerability, it may be a good idea to bid 2♠ as it prevents opps from balancing with 2♥ and the maybe raising to 3♥.

[FelicityR]

Pre-emptive action is pre-emptive action so bidding 2♠ to transfer to a minor essentially means that the opponents have to bid or guess at the three level (except where a double of 2♠ shows ♠s, or 2NT has some conventional sense here.)

I'd rather play in a suit contract at IMPs than MPs. Obviously being in 1NT at MPs where 8 or 9 tricks are available in both NTs or a minor is a better score.

Other than that, forget about 2♠ being just a weak takeout: It can be used with hands that are strong, too. The opponents are not going to know that until you pass or correct or bid a new suit, and then again they will have to make a decision or a guess at the three level.