And exactly what purpose do they serve?
Intercepting Stealth bombers?
MiG-29s and Su-27s have a little-known tracking device called the OEhPS, or Electro-optical Search and Track, sometimes known as Infrared Seach and Track. This device can detect the heat of an aircraft and is not bothered by radar-jamming or minimalization.
It's biggest benefit is that it's passive, emitting no signals that can warn its target. Those old movies where the pilot starts yelling "He's got lock on!" refer to radar target acquistion, but the OEhPS gives no warning until a missile comes up your exhaust pipe.
ANY escalation of military technology in Latin America is bad. For the US, and for Venezuela's neighbors. FIFTY modern interceptors is a big worry, much more than far-away Cuba's half-dozen.