To put it simply, it's up to the individual, in India, there are even schools that have 1 yr Diploma in Aviation & Hospitality Management from which airline companies do mass recruitment. One in Malaysia that has tie-ups has a 2.5 month course to train on deportment, grooming, socialisation, etc. Airline recruiters fly to these schools and recruit candidates - Something like on campus recruitment. Chances will be better.
In Singapore, the centres are sprouting up with 1-2 day course will prep you on the interview stages, what to do, what to wear, and a mock interview - It's up to you whether you apply it.
EK screening days aren't like the most, they can afford to be picky. If you notice from some recruitment days, we have candidates flying from Korea to Singapore - Why? - They aren't students of the agency which the airline recruits from in Korea and have difficulty gaining access there.
So far I know 3 schools now sprouting up - Nothing guarranteed, they just coach you. None have tie-ups, I know Tiger Airways used Red Dot.
Sorry about not making it with Emirates, you can try again. Yes, it's an investment - It all depends whether you have extra cash, but if not, some of the things are inherent within you - Grooming skills, confidence, interpersonal skills, cultural and customer service orientation, etc - Different airlines have different stringent requirements. SQ may have 7 rounds, EK - 3 to 4, UA - 3, MI - 5 odd, etc.
I wouldn't advise going for such courses unless they have tie-ups as their program will be geared towards that particular interview. Otherwise, take it as a form of self enrichment - These courses range from $190-300. Look at their candidate endorsements - Do you see many ex-crew graduating from the institute? If you go and ask around - you'll notice very few crew actually took these courses up before applying. There are many people wanting to be crew so there is a market demand for such courses. If you have extra cash and have the time, go straight ahead. But generally the content will be similar
Most of us never went through such courses - I agree with SingaporeGirl's comment on failures lead to success. But in a nutshell - If you got the cash and just want to practice for $190-300 for a 2 day class with things you probably would already know but want to confirm or practice, then go for it! Just don't feel cheated or angry when you spend this money and realise when you go for a CC interview that it really didn't matter anyway and didn't get selected again.