Tampa Bay does have Benn, but Tate should be good enough to be #3 there, if not compete for #2 WR.
2010 Stats:
Arrelious Benn: 15 games, 25 receptions, 395 yards, 15.8 ypr, 2 TD, 1 fumble
Brandon Tate: 16 games, 24 receptions, 432 yards, 18.0 ypr, 3 TD, 1 fumble
After Benn they only have Sammie Stroughter (24 rcpt, 239 yards, 0 TD), Michael Spurlock (17/250/2) and Dezmon Briscoe (6/93/1); add in Josh Freeman at QB and that's probably as good a situation as he could ask for.
St. Louis may indeed sign him but if he goes there I think Tate will still be buried on the depth chart, behind Amendola, Gibson, Avery and Sims-Walker.