Hey Nate,
I’m the adventurous type so my personal opinion is that if you spent the time to grow out a seedling, you might as well let it fruit to see what you have. Who knows, you may chance upon a new flavor profile or just get super lucky with an amazing tasting fruit.
It is highly unlikely that you will get a super amazing fruit but there is a good possibility that you get a decent tasting fruit. My friend Margot grew out a tree from seed about 20-25 years ago and here tree is very productive with good tasting fruit. It’s not top tier but it’s a good fruit in the same league as Kent, Glenn, Vp etc...
http://tropicalfruitforum.com/index.php?topic=21350.msg261001#msg261001Leo Manuel also planted out many seedlings and he has made a number of selections from them. Not all the seedlings were keepers but the ones he kept are quite good and very disease resistant.
http://tropicalfruitforum.com/index.php?topic=3188.0Nate, not all seedlings are the same. With Polyembryonic mangos, there is a significantly higher chance of good quality fruit from the seedling because there is a high probability of getting a clone. With Polyembryonic mango seedlings, there is also the probability of getting the zygotic seedling but the zygotic seedling may be selfed, meaning it was pollinated by itself.
This selfed seedling is Not a clone even though all of its genetic material came from itself. The zygotic seedling is the result of sexual reproduction so there were rearrangements of its genetic material. This is a possible explanation for how Lemon Zest and Orange Sherbet were selected from Po Pyu Kalay seedlings.
I highly recommend that us SoCal mango growers grow out Polyembryonic seeds from varieties like Sweet Tart, Orange Sherbet, COC and NDM as their number one choice for rootstocks. I recommend these even over Lavern Manilla.
Monoembryonic seedlings are also excellent as rootstocks but you are much less likely to get excellent fruit from them. Monoembryonic seedlings are highly variable from the research I have done. Monoembryonic seedlings usually start out as bigger plants as soon as they sprout because they get the energy fro the whole seed unlike Polyembryonic seedlings.
Simon