Especially in BG2 (because of the higher level cap and level requirements), I found that if I recruit a character early, change it with someone, and will need him/her later, they will be horribly underleveled compared to the rest of the party. Does that mean that once I have recruited someone, I should decide NOW, to either keep him/her forever or disband immediately? I ask this because I see no way of re-integrating old and under-leveled party members, as they don't gain any exp while not in the party.
nah. In d&d character exp is almost exponential, meaning, a high level mob you kill will give enough exp to a level 1 character to get to level 2, the next mob would take him to like level 3, etc. After a while the character will be about the same level as the others. Those 10 levels the others had over the new character would represent, say, 130k exp. To go to level 15 to 16 it would be like 1 million exp, so those 130k only means the new character will reach level 16 an hour later. (made up numbers, I don't have the exp tables with me, but you get the idea)
The answer by @Rodolfo is valid only for a very limited range of the game and does not apply to the second game in the series. While in the beginning XP costs tend to go up exponentially, the curve flattens out and even becomes linear towards the end.
Therefore there is a real disadvantage if you leave a character out for a longer period of time.
For example, for fighters the XP table becomes linear as soon as level 9 (start of BG 2).
Finally, to adress the question more directly: none spellcaster do not suffer that much from being a few levels behind. So it is not always downright suicidal, just suboptimal.