The short, canned response is "Yes, it's a fairly bad idea."
The longer, more accurate response is "It can very easily be a bad idea, and usually is, but there are "Ifs" involved that could mitigate it."
- IF all you have are consumer speakers and you can't afford to replace them with anything better,
- IF you are mixing for non-commercial purposes,
- IF your speakers have a fairly flat and extended response,
- IF you understand their sound well enough to know that if your music sounds like X on your speakers that it will sound like Y on many other systems so you can set your mix appropriately,
and
- IF it comes down between using them to make your mixes or making no mixes at all,
THEN
yes, it's OK to use them for mixing.
If any one of the above conditions rings false - and sometimes even if they're all true - you can expect to have trouble getting good response in your mixes in almost any part of the spectrum when you play them back elsewhere.
Can you take decent pictures with a $200 digital camera and ambient lighting? Sure, they'll be OK. But don't expect them to look like they were taken by a F-series Nikon with pro lighting. It's the same thing with sound. You can use some of the better-quality consumer speakers, but don't expect the results to sound like the professionally-made CDs in your CD changer.
G.