to me, to a certain extent, breeds ARE linebred, that's what makes them breeds.
the only value placed on any animal is what the market dictates.
the debate is still out there what the relative merit of these genes in what backgrounds have. there undoubtedly will be more tenderness and marbling genes discovered.
there a few angus breeders doing exactly what you mention. there are some criticisms, such as lack of spring of rib, small SC and fertility questions, smaller ribeyes etc.
the best thing about it, is you can at least track traits in a more measurable fashion, check your progress in fewer generations than was possible with conventional methods. there are a few Maine bulls with these traits, and some cows, but haven't seen much buzz about it yet. the only buzz i have seen get repeated is the T1 gene when polled energizer was advertised with it by derouchey's. he seemed to not have much credence in one of the marbling genes, but then with 2 available and 2 carcass traits, at least one can see if they have merit by tracking results as oppossed again to just concentrating lines and praying. the more these traits are utilized, the quicker we will see if they have merit. i'm all for it. it will spur more research, more checking, validation, invalidation etc.
downsides might be the incentive to create new fields of markers rather than more markers within existing fields of tenderness and marbling and the marketing plan for revenue takes too large a foothold at the expense of reality for utilizers of the technology.
there is one bull SAR he's a lad 269M (323682) who is homozygous for T1, T2, unknown for T3, but i think he is at least hetero. he's black, but not homo patterened. as mentioned before, total package 1p is at least hetero for 3 tenderness genes, homo for probably 2 of them. he's thrown a few 6 star individuals, only meaning that the mother gave her 3. a few 6 star individuals sold at derouchey's last year. using his catalog, it's pretty easy to see who has what. i may be wrong on this, but ali has thrown a 6 star heifer, but since he's probably heterozygoous for the tenderness genes, he's not thrown his stars by chance. Major's money man is also at least heterozygous for the 3 tenderness genes. T2 is the least common of the 3 markers from bovigen.
let's go maines, test for those genes and get some carcass data out there!
as far as value, if costco, walmart are the biggest outlets for beef, and they don't market marbling or tenderness, then 50% of the market is left after that, which may be sufficient to allow one to make $ with a premium carcass. so far it's worked for me with my measly two steers sold as "no additives, no hormones, no ionophores, no antibiotics, with free access to pasture, hay and grain."