Since the promotors are the ones who make the initial investment and take the initial risk, they are offered certain rights that normal shareholders do not enjoy. For eg, as you might have noticed, most of the Board members are from promotors group, and understandably so because this encourages initial investment and' arguably, entrepreneurship as well. So the differentiation is there for a reason.
For returns point of view for long term investor, both the shares should be the same. One might also argue that the promotor share should cost more because of the added value, but then again, the price is determined by the market. To be honest, I am a bit surprised that promotors share cost less - and not more - than ordinary shares in our market.
This should answer the initial issue.
Now about the influence of Board on Management, if both sides followed the written policy in a disciplined way, bananking industry in Nepak would be in a far better shape than it is now. It is easy to point your fingers to the Promotors but the Management/CEOs in most banks are equally to be blamed because most CEOs would do exactly what a Board member/Chairman tells him/her to do instead of following set guidelines and using proper judgement. The case of ex MD of KIST Bank is just the tip of the iceberg.