I had the same input regarding the X number of free messages a day. We also started as free memeber and probably would not have become paid members under the current restrictions.
As we all know, you can get and respond to dozens of messages that do not lead to any actual meetings or play time. The proof is in the pudding, but as the song lyrics go, "how can you have any pudding if you don't eat yer meat". In this case, "eat yer meat" = pony up some $$$.
Some will say you can sign up for 1 month and cancel if it doesn't work out, but many (myself included) are reluctant to offer up a credit card for any sort of temporary membership, and probably more so when it's for a swinger website.
They do need to make free users feel some pain because there'd otherwise be no reason to ever sign up. Even 1 or 2 free messages a day would give a better idea if it's worth shelling out any money for a membership.
Of course you will have the people that will send a message with an outside email address, and it may not be worth the effort to monitor that and/or rewrite code to enforce the message restrictions.