I think as has already been said that when a lot of people say, 'I'm so OCD' they mean it more in a trivial sense and are not genuinely declaring they suffer from this often devastating illness.
I have, in the past, suffered from severe bouts of OCD but have to disagree with you on the idea of OCD not varying on a day-to-day basis. Certainly, I have found that whilst my OCD will not change on a daily basis, I certainly have worse and better periods. For example, when under extreme stress, OCD will usually become worse. Sufferers may find they feel OK for a few months and then life becomes more stressful and the compulsions begin again.
Personally, I'm in two minds about people casually saying they have OCD. On the one hand, I do know what you mean and sometimes it can feel like people are making fun of a serious condition. However, it always draws attention to what was once a barely heard of condition and anything which encourages people to discuss mental health or make others aware of it it a good thing. Take this thread for example, people are now discussing OCD. Honestly, I don't think people are intentionally trying to be offensive.
Final point: every joke has an element of truth. Perhaps some people do genuinley feel they have the condition and are broaching it light-heartedly to see how people react. Also, most OCD sufferers will hide most of the behaviour/suffer a constant battle in their head. OCD comes in different severities. Mine is (was?) extreme but others will only have a very mild version...
Oh also, anyone suffering OCD who needs someone to talk to can feel free to PM me. I know just how isolating it can be, you are not going mad and things will improve. It is difficult but not impossible to get better.
ANYWAYS, that's my thoughts on the topic. Good thread OP