Speaking personally, I wouldn't do that.

Miles, could you explain your thoughts?

It's not really a question of morals but rather rights.

You sold your client a logo, not a (limited) number of glyphs. In other words: you are free to expand your designs to a typeface.

I agree Bert, but I wonder what is underlying their trepidation.

Is the logo just their name set in type, or is there also a graphic symbol?

Whether it's legal or not probably depends on your contract with the client. Since many logos incorporate pre-existing fonts, your contract probably doesn't guarantees font exclusivity, does it? But if the logo is just type, or if the font was very distinctive, it would likely piss off your client if you make it into a font that anyone (even competitors) can use, since that greatly reduces the uniqueness of their logo. And a pissed off client might sue you and/or start saying bad things about you.

I think it all depends on what you have agreed on with the customer. When you told the customer that you made him something special and exclusive, I think he’d be surprised at least as soon as he sees headlines in the very same style that were made using your typeface. Just imagine someone using it for a similar trade or product … So when you are still on good terms with that customer I would not take the risk in spoiling the relationship.
No matter wether you have sold the customer exclusive rights to use that logo or not, when your customer and his lawyer are willing to see that you have been unfair or enabled others to mimick ‘his’ logo or whatever, you will be likely to spend money on a lawyer as well. In many countries, design protection is not precisely or properly (if at all) regulated or governed by law, so it usually all depends on how such a case is handled, defended etc … I think, this is not necessarily the kind of things designers like to occupy themselves with, let alone paying someone else to take care of that.
So when you sell something as a designer, not only make shure that you are getting paid reasonably for the job, also make clear agreements on which rights on your work you are selling to the customer.

Something else to consider: the chance that the original glyphs remain the same during the process of adding to them to obtain a full set is rather slim. You will probably change most of them because they won’t fit the newer ones, or you change your mind about certain design decisions.

So why bother so much about something that is not a ‘real’ problem?