I will admit some simpaty, say 37% ::). Assuming IMP scoring, being vul against not, partner is not kidding around with his bid. He most certainly have 5+ diamonds, maybe just 4 in major suit, and decent hand. Its not impossible at all to make 5 diamonds. Even if we are not making it, maybe it is good 1-down save from their game in other major (if they happen to have it), and maybe we will even push enemy to level five. However, I would appreciate to cooperate with partner in making decision, so 4 diamonds (or 4 clubs, if artificial invitational raise in diamonds) seems better on long run...