It depends on what game mode. For defense, I place a valentina in a totem and keep cupid in, or vice versa based on your preference. For guild wars I use cupid. For Lost Realm she is good paired up with your aries team, as she removes calamity and therefore can take out the oposing aries team. If you are looking for quick damage increase then use cupid, this is good for dungeons. In all cases, she is an alternate to cupid, but not a replacement. I have used cupid and valentina together in dungeons, it is a devestating affect, as you have damage increase and your team goes full speed very quickly.
As Gob said yes you shall use her and it all depends on gamemode if you need cupid or val or both. In dungeons cupid and val proc together make your PD proc more often which make it easier to clear the dungeon. You will have to replace Aries for Val then for dungeons.