Risk of setting a precedent by allowing Iranian enrichment is less than the alternative
Skeptics, though, argue that the P5+1’s negotiating position—which envisions a final deal that would allow for a limited Iranian uranium-enrichment program—would undermine nonproliferation by encouraging other states in the region, especially Saudi Arabia, to pursue an enrichment capability. While a deal is highly unlikely to prompt Iran’s regional adversaries to make a mad dash to actually acquire nuclear weapons, the concern is that unless Iran’s nuclear program is dismantled, regional states will build their own domestic enrichment facilities, thereby placing them closer to being able to build nuclear weapons in the future.
However, the concern that an agreement allowing some Iranian enrichment will encourage the practice elsewhere is overblown. The alternative is no comprehensive P5+1 and Iran nuclear deal, which would result in a less constrained—if not unconstrained—Iranian nuclear program with less monitoring. This poses more of a threat to regional security and would be more likely to increase the possibility of a cascade of regional fuel making.
Several states, notably Saudi Arabia, have been vocal in their opposition to the nuclear deal with Iran and have argued that they will seek to pursue a similar enrichment program to the one Iran is pursuing, raising the spectre of an 'enrichment arms race'. However, empirically, states have threatened to pursue nuclear programs in response to other states but the cause and effect dynamics are rarely that simple, with states pursuing weapons programs (or enrichment programs) for a number of different motivations.