Ideally, I believe Baptism should be the mode of reception for converts, at least in our time (in addition, this is the manner converts are received if "exactitude" is being practiced by the Bishop). I believe this because...
a) There are, sadly, plenty of false witnesses in our day who in the name of ecumenism, are teaching a "baptismal theology" which confuses the real borders of the Orthodox Church, and tries to muddy clear waters. It is simply erroneous to equivocate between the mysteries celebrated by the Church, and those celebrated in non-Orthodox confessions. Baptizing converts clearly refutes this modern error in practice.
b) There are sadly, many cases where non-Orthodox are being received from confessions that administer something which can scarcely be viewed as a baptism of any kind (water simply being rubbed on a baby's forehead, or non-trinitarian baptisms, or strange forms of baptism practiced by some sects.) This situation could be avoided if exactitude was practiced.
c) I think there are some converts to Orthodoxy, who in latter years have some anxiety over how they were received. While this may be unfounded (since reception by chrismation is permissable), it's better to avoid such scruples, particularly if point (b) may apply to their situation.
I'm sure there are other reasons I could think of, but those are the most immediate.
I of course do not doubt for a second the propriety (when the motives are good) of receiving a convert via chrismation - I know St.Herman of Alaska had received a Lutheran convert this way. As another poster mentioned, this is how Fr.Seraphim (of blessed memory) was received, under the supervision of St.John (Maximovitch.) Though I am a part of a "juristiction" which receives converts by baptism (exclusively as far as I know), it is mostly for the reasons I just mentioned, and not for the following reasons (which I find to be wanting, and historically inaccurate)...
1) Anything besides baptism by triple immersion cannot even qualify as having the form of a baptism: though I agree it is wrong to make baptism via pouring some water over the forehead the "standard" (even in the west this was a late in the game innovation, which began centuries after the "great schism"), the truth is that the Didache (ancient Christian manual from the late first/early second century) says this is an acceptable form of baptism if immersion is not possible, and it is done in cases of emergency even in the context of modern Orthodoxy. Also, there is some evidence that something like a combination of immersion/pouring was practiced in some parts of the ancient western church (person entering a shallow baptismal pool, and having water picked up from it and poured over his head).
2) Heretical/Schismatical Sacraments are not true mysteries, so they must be ignored: While I agree they are not true mysteries, this has not stopped the Church from filling and healing what is wanting in them in times past. If one denies this possibility, then they are going to have to argue with quite a few Saints, since a more liberal form of reception of converts from other Christian groups has, on the whole, been the norm in the Orthodox world (particularly in Russia).
With that said however, I think there is an important value in receiving converts via exactitude; but if the motives are proper, it's not some great travesty of justice for converts to be received via chrismation (or in some cases, perhaps if the entire form of the rites they received in a sect were entirely Orthodox, by something as simple as a renunciation of errors and profession of the true faith.)