All very valid points.
For me, it runs the other way round as well. About once or twice a month, I get someone with a web app they are developing looking for me to write their copy and do some PR/Marketing work for them in exchange for a profit share.
Often these people underestimate what I charge for these services and how long they take to do, so they labour under the false assumption that I’ll be biting their hand off for 10% of their profit in exchange for ‘doing a bit of writing’.

My favourite example was a student who had no budget but wanted me to sign a contract to say I wouldn’t ever sue him to get more than 7% (!!!) of the profits when his idea went stellar and that – and that the maximum profit I could earn was capped at £750. Needless to say I politely declined.