John of Japan said:
I disagree. Paul only worked when he had to. Note that Paul was usually supported by churches in his missionary work (Phil. 4:15, 2 Cor. 11:8, etc.). He only had two reasons for working at his secular profession: to avoid being indebted to the Corinthians he was trying to reach for Christ (as you point out), and to make ends meet when the churches were not faithful to support him.
Brother John, I must also disagree. It seems your use of "when he had to" is restricted too narrowly. At least a third reasons exists, as found in Acts 20 and II Thessalonians 3 -- as an example.
Acts 20:33-35 I have coveted no man's silver, or gold, or apparel. Yea, ye yourselves know, that these hands have ministered unto my necessities, and to them that were with me. I have shewed you all things,
how that so labouring ye ought to support the weak, and to remember the words of the Lord Jesus, how he said, It is more blessed to give than to receive.
II Thess. 3:7-9 For yourselves know how ye ought to follow us: for we behaved not ourselves disorderly among you; Neither did we eat any man's bread for nought; but wrought with labour and travail night and day, that we might not be chargeable to any of you: Not because we have not power, but
to make ourselves an ensample unto you to follow us.
I think the overall picture indicates Paul and others worked more often than not, and even when they received help from others it does not mean that it was some type of full salary that keep them from working. In his writing to the Corinthians, we learn that there were two parts combined to keep Paul from being chargeable to them -- working and support from other churches (e.g. cf. II Cor. 11:9)
Though in I Cor. 9 Paul indicates that Peter and others did accept their maintenance (and even their wives, 9:5), it does not follow that they always did so. I Cor. 4 indicates that other apostles at one time or another labored with their hands. Note that "we" is the subject of the sentence in verse 12, and "apostles" is the antecedent of "we".
I Cor 4:9-12 For I think that God hath set forth
us the apostles last, as it were appointed to death: for
we are made a spectacle unto the world, and to angels, and to men.
We are fools for Christ's sake, but ye are wise in Christ;
we are weak, but ye are strong; ye are honourable, but
we are despised. Even unto this present hour
we both hunger, and thirst, and are naked, and are buffeted, and have no certain dwellingplace; And
labour, working with our own hands: being reviled, we bless; being persecuted, we suffer it:
So Paul was not the only one for whom this was a practice. He cites Barnabas in I Cor. 9, verse six (to me it appears this reference to Barnabas is after he and Paul had separated; cf. Acts 15:36-41 & 18:1-17). In I Corinthians 12, he indicates that Titus also followed him in this.
Paul's argument of chapter 9 in I Corinthians seems to be a little flat if he was not making this a common practice but was doing it only because he had to. He labored not only in Corinth, but also in Ephesus and Thessalonica, and not only because he had to but to set an example for others. And in I Cor. 9 he is not talking about something he was forced in to by not getting support, but a right that he had in a deliberate manner set aside.