I find that, if I inadvertently write an overlong text message, I can often reduce its size enough to get it back to being an SMS rather than an MMS by quick and obvious editing. Otherwise, I think the phone's cut and paste is good enough now for this to be a decent workaround... I simply select the second half of the message, cut it, send the abridged SMS and then send a second SMS with the cut text pasted into it. Not perfect, but ok.