As a native English speaker who teaches English to Spanish speakers presently, and has non-native friends: the grammar mistakes that make it challenging to understand non-native speakers, are when the verb is not properly associated with its direct object. 

Recent experience of a sentence with poor grammar:

"She can to the store, walking for 10 minutes." 

It was hard to understand if they meant:

It took 10 minutes for her to walk to the store. 

She walked around in the store for 10 minutes. 

She can only handle walking for 10 minutes at a time.

If you are able to associate the verb with its direct object then even if there are other mistakes, you will still be understood. 

Different people have different goals. For me, the first goal is communication. While it is probably a good idea to strive to learn "good grammar" with your target language, mistakes in grammar--and pronunciation--that don't interfere with understanding are not as serious as those that do.

I have a second thought, which is that it's sometimes harder to learn and use the "incorrect" informal colloquial grammar properly, than it is just to learn the correct grammar.

"Incorrect grammar of everyday speech" is "easier" than formally correct grammar for native speakers, because they grow up with it and it's what they hear more hours than they hear "classroom" speech. It probably isn't easier for learners. 

It's also surprising just how often you need fairly sophisticated grammar in everyday life. Just today, I wanted to tell a language companion:

"I think I will be able to talk with you today at 1:30 p.m. but I'm not sure. I've just set my Skype status to 'absent.' If I'm waiting in a doctor's office I won't be able to communicate easily with you. When I know for sure that I can make it, I'll set my status to 'green.' If you don't see green at 1:30, don't wait for me."

That's really quite an exercise for my Spanish grammar! Negative imperative, condition, subjunctive, future... and all in a very ordinary everyday message.

Of course, I could say

"I think I is able talk today at 1:30 but not I is sure. I make my Skype status 'absent.' If I wait in doctor's office I not can communicate easily with you. When I know for sure that I make it I is set my status 'green.' If you no see 'green' at 1:30, you no wait." And it would be understood.