Originally posted by: .Vrish.
In Krishna's case, each of his major wives had 10 sons. In the case of the 16,100 wives he captured after killing Narakasura, the reason he married them was that they had no future: no other man would marry them b'cos they had already been violated by Narakasura.
Pradhyumna marrying Rukmavat and Anirudha marrying Rochana were unhealthy, given that Vidharbha was already an ally and bound to Dwarka due to Rukmini alone. Same w/ Arjun & Subhadra: Dwarka was already beholden to the Pandavas due to Kunti being Vasudev's sister. Subhadra's marriage to Arjun was to prevent the alliance from being trumped if she married a Pandava enemy, like Dury. Also, as Shalya turned out to prove, being an uncle of the Pandavas didn't prevent him from joining their enemy.
Chitrangada was a genuine romantic interest of Arjun's, and did nothing for the Pandavas politically: Arjun's son Babruvahana was totally out of the Pandava succession, and even though he was king, he didn't participate in the Kurukshetra war, even though his father was there in it. Also, Manipur was nowhere near Indraprastha, and Pragjyotisha - Guwahati - was a Kaurava ally.