Generalisations are excellent, they're ultra useful for short explanations.
The problem is, lots of people assume that by generalising, you are steriotyping. Or that you treat a generalisation as fact for everyone. That's the reason they don't like them. Personally when someone generalises, i assume they are generalising, which an extremely usefull skill and tool in life.
For example, where i work, some generalisations i make of people in the factory are: a lower IQ, lower education level, feeling uneasy/vunerable in the office, poor understanding of computers, poor understanding of taxation, rate calculations and terms like cumulative and gross/net.
All of these make me better at my job by targeting my responces. BUT, someone else will come along and say "but for all you know they could have a masters degree and be cleverer than you, you shouldn't generalise". They think they're being clever but they're not saying anything i don't already know. Of course they could, but the majority will not, therefore my responce is tailored to the majority. If they show themselves to be more comfortable with the discussion i will change my language and explanations to my new assumed level of knowlege.
Generalisations are excelent, as long as you use them right. Don't let other peoples poor abilities unfluence you against them.