I second dan_and. Very nicely put.
An immigration consultant comes in handy if
- you don't have time
- you are afraid you might miss something
I got an application returned to me in 2014 for missing documents, which was basically a rejection because I received the returned package in 2015 so my chances of re-sending a complete application was gone, simply because 2014 applications were closed on December 31, 2014.
In 2015, my ONIP (Ontario provincial nomination) application was also returned to me for being incomplete. This time I was lucky enough because Ontario used a courier service so the package arrived only in 3 days, giving me lots of headroom to complete and re-send the application.
I fumbled the federal application after ITA too. Thankfully, there is this forum and something called CSE, which granted me the chance to set my application right.
I am a reasonably intelligent person. The problem wasn't that I was an idiot. I just happen to operate my own business which requires extreme levels of attention. It drains all my energy at the end of the day. Despite I checked everything like 10 times, I was still able to miss small things. Small things have very big consequences in this kind of stuff.
So my point is, there is no shame or no wrong in getting some consultancy service. Actually, if I had to do it all over again, I would use one.
Consultants not doing their jobs properly is another matter. You just need to find one who will.