Opinion and exit polls are two separate entities and they have to be analyzed separately in order to know their pros and cons. Opinion polls are tools that are used to sense the mood prior to the elections. Since we are a democracy, electing individuals who will not be a part of the government formed will not be advantageous.
In a country like ours where the population is in billions one cannot comprehend the mood of the election without devices such as opinion polls. Considering the latest example of Delhi election, one can give a thought about how things would have changed if pollster had not predicted that AAP will also be a force in the election.
What if the opinion poll result created a sense of confidence among voters that others are also voting for the same party? Of course overtime it has been quite clearly proven empirically that bandwagon effect (backing of dominant candidate) prevails over the underdog effect (underdog gets sympathy vote).
So opinion polls might reduce the occurrence of a hung assembly which usually results in inefficient governance.
The argument by those insisting on ban on opinion polls is that they are inaccurate. On certain occasions this has been true. Classical examples include 2004 loksabha election, UP assembly elections. There are indications that social media content (twitter, FB) is used for predicting voting patterns. The major defect with this approach is that it includes the sample population having access to internet, computers, and smart phones. It does not include voters without access to twitter or FB. However if the regular prediction models are coupled with these new models we might have more accurate predictions based on which the voter can decide smartly.
Another reason which gives negative idea about these opinion polls is that it can be biased. The media as it has been proven many times can be prejudiced or sometimes even bought for money or other favors. There has to be some regulation under which these polls have to scrutinize but enforcing a ban will not be ideal. It is illegal to have opinion polls in Bulgaria but the media gives out ranking of the most popular street of a candidate or a party thereby predicting the result as well as not breaking the law.
Political mileage can be obtained if an opinion poll is backing the party. It's only the indoor type politicians who are worried about the opinion poll's results, the serious politicians know that it requires a lot of hard work to win an election.
It is also to be noted that due to by election commission's restriction in holding political rallies that such tools like opinion polls have gained significance.
Now considering exit polls, there is nothing much to worry about it. EC has a ban on exit polls until 48 hours of the last hour of voting. It is more to satisfy the curiosity of the voter and the political parties rather than influence a voter. The same reasons exist for inaccuracy of the exit polls. Inappropriate and irrelevant sample sizes, not considering the demography of the constituency exactly and a basic 3% error when extrapolated to a larger population are the reasons why they have failed previously.
Like all the improvements happening in science & technology, psephology is also undergoing improvements. Errors are inevitable but the margin of error is decreasing over the period of time.
Certain regulations have to be enforced to make these polls unbiased but a ban on them is definitely not the solution.
Opinion poll provides an opportunity for the voters to cast the vote intelligently rather than follow some blind way that has been prevailing all these years.