We don't know what the Romulans gave up on their end of the deal. For all we know, it was to further develop those super-powerful plasma torpedoes they used in "Balance of Terror" against the Federation.
As it is, it's not such a bad deal. The Feds stop developing cloaks of their own, but they can still develop anti-cloaking sensors which would render cloaking useless (especially since cloaking takes away shields and weapons).
As it is, it's not such a bad deal. The Feds stop developing cloaks of their own, but they can still develop anti-cloaking sensors which would render cloaking useless (especially since cloaking takes away shields and weapons).