I'm registered legally and I will tell you why.
1. I have my own office with public access (not working out of my home), so need to be registered in case of workplace inspections.
2. I also do translations and cannot bill a company unless I am legally registered.
3. A number of my clients have their English classes paid for by their employers and need an official invoice. Again, need to be legally registered to do so.
4. My clients can pay by credit card. The only way that is possible is being legally registered since there is a definite paper trail and the need to file taxes.
As has been discussed on other threads of this forum, registering legally apart from being the responsible thing to do (I am not getting on a soapbox or going to be preachy

) is a personal decision.
I agree that it is a costly thing to do, especially in the beginning when you are establishing yourself (and believe me, after 3 years, I am still stablizing the business... need at least 5 to do so). Like the saying goes, you have to spend money to make money. However, it was probably the smartest business decision I have ever made. Now based on my income growth over the past few years, my bank has extended me a 9,000€ line of credit to use at my discretion.
Also based on our joint income, my husband and I were approved for a REASONABLE mortgage. And anyone who has been following the real estate trend here in Spain (especially Madrid) can attest to the challenge that THAT is.
Tim Horton's Man, I think that the majority of the English teachers (native speaking) who are here, aren't planning on being here for more than a couple of years so don't feel the need to register themselves (and pray that they don't get caught...). However, there are those of us here who plan on being here for a good while and would like to earn a pension (through social security payments, etc) and establish themselves more in the society.
Just another perspective for you to think about!